Shareholders' Equity (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Components of AOCI
Net unrealized loss on securities available-for-sale	$ (1,749)	$ (1,749)		$ (1,749)		$ (7,070)
Foreign currency translation adjustments	(11)	(11)		(11)		(4)
Total accumulated other comprehensive loss	$ (1,760)	(1,760)		(1,760)		$ (7,074)
Securities available-for-sale:
Net unrealized (losses) gains arising during the period		(11,011)	$ 16,820	13,238	$ 38,348
Reclassification adjustment for net gains included in net income		(306)	(5,359)	(3,691)	(7,551)
Non-credit related impairment loss		(363)	(317)	(363)	(317)
Total securities available for sale		(11,680)	11,144	9,184	30,480
Foreign currency translation adjustments		4		(7)
Total other comprehensive (loss) income		(11,676)	11,144	9,177	30,480
Components and related tax effects of other comprehensive income (loss)
Net unrealized (losses) gains arising during the period		(4,605)	7,056	5,559	16,031
Reclassification adjustment for net gains included in net income, taxes		(128)	(2,242)	(1,544)	(3,159)
Non-credit related impairment loss, taxes		(152)	(133)	(152)	(133)
Total securities available for sale		(4,885)	4,681	3,863	12,739
Total other comprehensive income		(4,885)	4,681	3,863	12,739
Components of other comprehensive income (loss), net of tax
Net unrealized (losses) gains arising during the period		(6,406)	9,764	7,679	22,317
Reclassification adjustment for net gains/losses included in net income		(178)	(3,117)	(2,147)	(4,392)
Non-credit related impairment loss		(211)	(184)	(211)	(184)
Total securities available for sale		(6,795)	6,463	5,321	17,741
Foreign currency translation adjustments		4		(7)
Total other comprehensive (loss) income		$ (6,791)	$ 6,463	$ 5,314	$ 17,741
Common shares repurchased
Total number of Shares Purchased	628	628
Average Price Paid per Share (in dollars per share)	$ 92.62	$ 92.62