Shareholders' Equity (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended			3 Months Ended
	Mar. 31, 2015	Feb. 28, 2015	Jan. 31, 2015	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Components of AOCI
Net unrealized gain (loss) on securities available-for-sale	$ 5,045			$ 5,045		$ (7,070)
Foreign currency translation adjustments	(14)			(14)		(4)
Total accumulated other comprehensive income (loss)	5,031			5,031		(7,074)
Securities available-for-sale:
Net unrealized losses arising during the period				24,477	21,528
Reclassification adjustment for net gains included in net income				(3,386)	(2,192)
Non-credit related impairment loss				(228)
Total securities available for sale				20,863	19,336
Foreign currency translation adjustments				(10)
Total other comprehensive income (loss)				20,853	19,336
Components and related tax effects of other comprehensive income (loss)
Net unrealized gain (losses) arising during the period				10,260	8,975
Reclassification adjustment for net gains included in net income, taxes				(1,417)	(917)
Non-credit related impairment loss, taxes				(95)
Total securities available for sale				8,748	8,058
Total other comprehensive income (loss)				8,748	8,058
Components of other comprehensive income (loss), net of tax
Net unrealized (losses) gains arising during the period				14,217	12,553
Reclassification adjustment for net gains/losses included in net income				(1,969)	(1,275)
Non-credit related impairment loss				(133)	0
Total securities available for sale				12,115	11,278
Foreign currency translation adjustments				(10)
Total other comprehensive income				$ 12,105	$ 11,278
Common shares repurchased
Total number of Shares (or Units) Purchased	3,999	28,854	294	33,147
Average Price Paid per Share (or Unit) (in dollars per share)	$ 89.88	$ 89.91	$ 82.64	$ 89.84