Investment Securities - Investment Securities (Parenthetical) (Detail)	6 Months Ended		12 Months Ended
	Jun. 30, 2013 Y	Jun. 30, 2012	Dec. 31, 2012 Y
Amortized Cost And Fair Value Debt Securities [Abstract]
Federal statutory rate	35.00%	35.00%	35.00%
Weighted-average maturity of available-for-sale investment securities	5.4		4.1
Weighted-average yield of available-for-sale investment securities	2.72%		2.93%
Weighted average maturity of held-to-maturity investment securities	4.2		3.3
Weighted-average yield of held-to-maturity investment securities	1.89%		1.94%