Segment Information - Schedule of Financial Information Concerning Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net interest income	$ 79,191	$ 75,973	$ 158,241	$ 150,246
Provision for loan losses	(3,693)	393	(2,848)	2,449
Noninterest income	46,244	48,542	92,559	97,675
Noninterest expense	86,916	86,027	177,099	177,314
Income (loss) before income taxes	42,212	38,095	76,549	68,158
Total assets	9,641,071	8,689,556	9,641,071	8,689,556	9,543,623
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	79,191	75,973	158,241	150,246
Provision for loan losses	(3,693)	393	(2,848)	2,449
Noninterest income	46,244	48,542	92,559	97,675
Noninterest expense	86,916	86,027	177,099	177,314
Income (loss) before income taxes	42,212	38,095	76,549	68,158
Total assets	9,641,071	8,689,556	9,641,071	8,689,556
Community Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	67,192	66,866	134,597	136,298
Provision for loan losses	(2,446)	(3,466)	(2,801)	(3,720)
Noninterest income	15,867	27,212	30,321	53,948
Noninterest expense	43,937	61,005	90,851	122,824
Income (loss) before income taxes	41,568	36,539	76,868	71,142
Total assets	5,032,076	4,593,862	5,032,076	4,593,862
Treasury Funds [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	(3,006)	(5,508)	(6,612)	(11,898)
Provision for loan losses
Noninterest income	3,474	7,919	6,148	10,375
Noninterest expense	1,566	2,331	4,032	4,443
Income (loss) before income taxes	(1,098)	80	(4,496)	(5,966)
Total assets	4,054,985	3,608,140	4,054,985	3,608,140
Other Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	15,005	14,615	30,256	25,846
Provision for loan losses	(1,247)	3,859	(47)	6,169
Noninterest income	26,903	13,411	56,090	33,352
Noninterest expense	41,413	22,691	82,216	50,047
Income (loss) before income taxes	1,742	1,476	4,177	2,982
Total assets	$ 554,010	$ 487,554	$ 554,010	$ 487,554