Investment Securities - Investment Securities (Parenthetical) (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2013 Y	Mar. 31, 2012	Dec. 31, 2012 Y
Investments Debt And Equity Securities [Abstract]
Federal statutory rate	35.00%	35.00%	35.00%
Weighted-average maturity of available-for-sale investment securities	4.4		4.1
Weighted-average yield of available-for-sale investment securities	2.80%		2.93%
Weighted average maturity of held-to-maturity investment securities	3.6		3.3
Weighted-average yield of held-to-maturity investment securities	1.91%		1.94%