Investment Securities - Gross Gains (Losses) Realized on Sales, Maturities and Other Securities Transactions Included in Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Gross gains, sales transactions									$ 3,274	$ 746	$ 14,098
Gross losses, sales transactions									(947)	(2)	(44)
Gains on sales transactions, net									2,327	744	14,054
Gross gains, maturities and other securities transactions									293	10	662
Gross losses, maturities and other securities transactions									(133)	(9)	(38)
Gains (losses) on maturities and other securities transactions, net									160	1	624
Gains on securities transactions, net	$ 6	$ 157	$ (92)	$ 2,416	$ 643	$ 103	$ 7	$ (8)	$ 2,487	$ 745	$ 14,678