Fair Values of Assets and Liabilities - Valuation Assumption Ranges for MSRs (Detail)	Mar. 31, 2013	Dec. 31, 2012
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected prepayment	17.00%	20.10%
Discount rate	10.30%	10.40%
Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected prepayment	12.00%
Discount rate	10.00%
Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected prepayment	28.00%
Discount rate	14.00%
Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected prepayment	17.00%
Discount rate	10.00%