AMP Ltd suffered a fall in earnings in 2013, on the back of lower insurance profits and weaker investment returns, but the group's banking division made a strong contribution.AMP reported yesterday that, while group earnings were down 11 per cent, its banking division increased its operating earnings from A$62 million in 2012 to $83 million last year - growth of 33.9 per cent.AMP Bank's loan book grew by 7.6 per cent to $13.3 billion. Residential mortgage growth was 7.1 per cent, which was above system growth. Net interest income grew by 19.4 per cent.The growth in mortgages was largely funded by an increase in wholesale funding, including $1.15 billion of residential mortgage-backed securities. In November, AMP Bank established a $500 million warehouse facility with Bank of Tokyo-Mitsubishi UFJ.The deposit book grew by 1.5 per cent and the ratio of customer deposits to loans fell from 69.9 per cent to 65.6 per cent.The net interest margin was 1.39 per cent - up from 1.23 per cent in 2012.The cost-to-income ratio was 29.6 per cent, compared with 34.9 in 2012.The bank's return on capital increased by two percentage points to 15.8 per cent.The loan impairment expense was 0.03 per cent of gross loans and acceptances.