Segment Information - Schedule of Financial Information Concerning Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net interest income	$ 77,996	$ 74,150	$ 236,237	$ 224,396
Provision for loan losses	(1,724)	400	(4,572)	2,849
Noninterest income	47,755	40,867	140,314	138,542
Noninterest expense	96,658	89,019	273,757	266,333
Income (loss) before income taxes	30,817	25,598	107,366	93,756
Total assets	9,652,079	9,383,044	9,652,079	9,383,044	9,543,623
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	77,996	74,150	236,237	224,396
Provision for loan losses	(1,724)	400	(4,572)	2,849
Noninterest income	47,755	40,867	140,314	138,542
Noninterest expense	96,658	89,019	(273,757)	266,333
Income (loss) before income taxes	30,817	25,598	107,366	93,756
Total assets	9,652,079	9,383,044	9,652,079	9,383,044
Community Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	68,513	69,613	203,110	205,911
Provision for loan losses	(915)	301	(3,716)	(3,419)
Noninterest income	18,225	27,785	48,546	81,733
Noninterest expense	53,677	67,085	(144,528)	189,909
Income (loss) before income taxes	33,976	30,012	110,844	101,154
Total assets	5,154,982	5,463,149	5,154,982	5,463,149
Treasury [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	(4,624)	(6,731)	(11,236)	(18,629)
Noninterest income	2,872	4,456	9,020	14,831
Noninterest expense	2,026	2,616	(6,058)	7,059
Income (loss) before income taxes	(3,778)	(4,891)	(8,274)	(10,857)
Total assets	4,008,279	3,460,547	4,008,279	3,460,547
Other Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	14,107	11,268	44,363	37,114
Provision for loan losses	(809)	99	(856)	6,268
Noninterest income	26,658	8,626	82,748	41,978
Noninterest expense	40,955	19,318	(123,171)	69,365
Income (loss) before income taxes	619	477	4,796	3,459
Total assets	$ 488,818	$ 459,348	$ 488,818	$ 459,348