Shareholders' Equity and Earnings Per Share Shareholders' Equity And Earnings Per Share (Other Comprehensive Income Reclassified from AOCI) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
(Losses) gains on available-for-sale securities, net	$ (24)	$ (336)	$ 500	$ (369)
Interest on junior subordinated debentures	1,977	2,042	3,910	4,046
Income before taxes	70,126	63,031	133,161	119,371
Income tax expense	(26,295)	(24,490)	(50,278)	(46,330)
Net income	43,831	38,541	82,883	73,041
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Accumulated Unrealized Losses on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
(Losses) gains on available-for-sale securities, net	(24)	(336)	500	(369)
Income before taxes	(24)	(336)	500	(369)
Income tax expense	10	133	(196)	146
Net income	(14)	(203)	304	(223)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Accumulated Unrealized Losses on Derivative Instruments
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Interest on junior subordinated debentures	457	521	871	1,014
Income before taxes	(457)	(521)	(871)	(1,014)
Income tax expense	179	207	341	403
Net income	$ (278)	$ (314)	$ (530)	$ (611)