Shareholders' Equity (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2015	Jul. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Components of AOCI
Net unrealized gain ( loss) on securities available-for-sale	$ 2,528		$ 2,528		$ 2,528		$ (7,070)
Foreign currency translation adjustments	(28)		(28)		(28)		(4)
Total accumulated other comprehensive income (loss)	$ 2,500		2,500		2,500		$ (7,074)
Securities available-for-sale:
Net unrealized gains (losses) arising during the period			7,741	$ (16,386)	20,617	$ 21,646
Reclassification adjustment for net gains included in net income			(12)	(7)	(3,703)	(7,558)
Non-credit related impairment loss			(325)	(243)	(325)	(243)
Total securities available for sale			7,404	(16,636)	16,589	13,845
Foreign currency translation adjustments			(17)		(24)
Total other comprehensive income (loss)			7,387	(16,636)	16,565	13,845
Components and related tax effects of other comprehensive income (loss)
Net unrealized gains (losses) arising during the period			3,268	(6,839)	8,676	9,060
Reclassification adjustment for net gains included in net income, taxes			(5)	(3)	(1,549)	(3,162)
Non-credit related impairment loss, taxes			(136)	(102)	(136)	(102)
Total securities available for sale			3,127	(6,944)	6,991	5,796
Total other comprehensive income			3,127	(6,944)	6,991	5,796
Components of other comprehensive income (loss), net of tax
Net unrealized gains (losses) arising during the period			4,473	(9,547)	11,941	12,586
Reclassification adjustment for net gains/losses included in net income			(7)	(4)	(2,154)	(4,396)
Non-credit related impairment loss			(189)	(141)	(189)	(141)
Total securities available for sale			4,277	(9,692)	9,598	8,049
Foreign currency translation adjustments			(17)		(24)
Total other comprehensive (loss) income			$ 4,260	$ (9,692)	$ 9,574	$ 8,049
Common shares repurchased
Total number of Shares Purchased	2,897	1,609	4,506
Average Price Paid per Share (in dollars per share)	$ 86.89	$ 89.65	$ 87.87