Shareholders' Equity and Earnings Per Share Shareholders' Equity And Earnings Per Share (Other Comprehensive Income Reclassified from AOCI) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Gains (losses) on available-for-sale securities, net	$ 524	$ (33)
Interest on junior subordinated debentures	1,933	2,004
Income before taxes	63,035	56,340
Income tax expense	(23,983)	(21,840)
Net income	39,052	34,500
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]
Gains (losses) on available-for-sale securities, net	524	(33)
Income before taxes	524	(33)
Income tax expense	(206)	13
Net income	318	(20)
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	414	493
Income before taxes	(414)	(493)
Income tax expense	162	196
Net income	$ (252)	$ (297)