It has almost been a given in Australian banking that Commonwealth Bank will forever retain a stranglehold over the domestic market for retail deposits.With A$246.2 billion of household deposits sitting in its digital and ironclad vaults at the end of September, CBA commands more than 28.4 per cent of the market.The bank occupies a privileged slot in the banking industry, with its ability to source oodles of relatively cheap cash to support its leading position in lending to homebuyers.However, an analysis of historical APRA data by Banking Day indicates that CBA's grip on most segments of the deposits market is starting to loosen.Based on trends apparent in local deposits markets, the bottom line for CBA's next chief executive is that he or she will be inheriting a bank losing momentum with customers.That is a stark contrast to the franchise that was firing on most cylinders when Ian Narev inherited the executive reins from Ralph Norris in December 2011.Since October 2014, CBA's share of household deposits has fallen by 0.5 per cent as major rivals - NAB and ANZ - along with challenger brands such as ING Direct have won market share.The APRA statistics indicate the negative trend has accelerated in 2017.Deep mining of the APRA database also reveals even greater market share losses for CBA in several niche deposit markets including the business segment.  In the last three years, CBA has grown its deposit base drawn from non-financial companies by A$17 billion to $107 billion.The growth, however, has only been sufficient to maintain CBA's market share at 20 per cent, while its fastest-growing rival in this deposit category, ANZ, has boosted its deposits by almost $30 billion to $94 billion.ANZ's gains in business deposits over the three-year period have lifted its market share from under 15 per cent to 17.4 per cent.Since August this year CBA has been haemorrhaging business deposits. In September business customers made net withdrawals of almost $3.5 billion, an outflow that coincided with the bank shifting retail businesses onto more expensive fee plans for using point-of-sale terminals.Trends in the business deposits market are a reliable indicator of where businesses are taking their overall banking relationships.Usually, there are big spin-offs for banks when business customers switch their transactional and savings accounts from another institution: loans, merchant acquirer deals and provision of company super are among the most lucrative.In 2017, small and medium enterprises increasingly moved their accounts to ANZ, Westpac and NAB at the expense of CBA.While CBA's historical advantage in retail banking means that it will retain its leading position among household customers for some time, NAB has already displaced CBA in special market segments with carefully targeted marketing campaigns.According to APRA, the not-for-profit sector was one of the fastest-growing segments for deposits in the last three years.CBA dominated community sector banking in 2014 when it accounted for $6.2 billion, or more than 28 per cent of the market.However, NAB now claims market leadership after nearly doubling its deposit base in the category to $10.3 billion.In