Loans and Allowance for Credit Losses - Allowance for Credit Losses (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
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 62,262	$ 60,732	$ 67,187	$ 61,234
Charge-offs	(16,136)	(3,749)	(26,612)	(8,741)
Recoveries	526	396	1,580	1,099
Provision for Loan, Lease, and Other Losses	10,800	4,297	15,297	8,084
Ending Balance	57,452	61,676	57,452	61,676
Ending balance: individually evaluated for impaired	9,655	11,734	9,655	11,734
Ending balance: collectively evaluated for impaired	47,797	49,942	47,797	49,942
Total	4,229,752	4,159,531	4,229,752	4,159,531	4,204,704
Ending balance: individually evaluated for impaired	66,826	80,590	66,826	80,590
Ending balance: collectively evaluated for impaired	4,162,926	4,078,941	4,162,926	4,078,941
Commercial Loan [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	20,275	18,143	19,852	18,200
Charge-offs	(13,683)	(1,754)	(14,221)	(3,668)
Recoveries	136	37	264	275
Provision for Loan, Lease, and Other Losses	9,847	2,876	10,680	4,495
Ending Balance	16,575	19,302	16,575	19,302
Ending balance: individually evaluated for impaired	6,627	8,046	6,627	8,046
Ending balance: collectively evaluated for impaired	9,948	11,256	9,948	11,256
Total	1,012,315	1,059,675	1,012,315	1,059,675	1,019,822
Ending balance: individually evaluated for impaired	24,592	31,271	24,592	31,271
Ending balance: collectively evaluated for impaired	987,723	1,028,404	987,723	1,028,404
Construction Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	7,733	6,427	8,928	6,756
Charge-offs	(671)	(150)	(755)	(340)
Recoveries	47	36	59	92
Provision for Loan, Lease, and Other Losses	1,137	1,688	14	1,493
Ending Balance	8,246	8,001	8,246	8,001
Ending balance: individually evaluated for impaired	267	2,747	267	2,747
Ending balance: collectively evaluated for impaired	7,979	5,254	7,979	5,254
Total	66,243	77,442	66,243	77,442	87,438
Ending balance: individually evaluated for impaired	3,904	14,915	3,904	14,915
Ending balance: collectively evaluated for impaired	62,339	62,527	62,339	62,527
Residential Portfolio Segment [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	5,749	6,702	5,908	8,237
Charge-offs	(321)	(742)	(643)	(2,454)
Recoveries	89	149	812	282
Provision for Loan, Lease, and Other Losses	922	510	362	554
Ending Balance	6,439	6,619	6,439	6,619
Ending balance: individually evaluated for impaired	1,943	431	1,943	431
Ending balance: collectively evaluated for impaired	4,496	6,188	4,496	6,188
Total	1,269,830	1,213,610	1,269,830	1,213,610	1,241,565
Ending balance: individually evaluated for impaired	10,468	2,911	10,468	2,911
Ending balance: collectively evaluated for impaired	1,259,362	1,210,699	1,259,362	1,210,699
Commercial Real Estate Portfolio Segment [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	18,470	19,371	22,441	18,961
Charge-offs	(694)	(306)	(9,238)	(541)
Recoveries	11	28	108	186
Provision for Loan, Lease, and Other Losses	(1,659)	(1,455)	2,817	(968)
Ending Balance	16,128	17,638	16,128	17,638
Ending balance: individually evaluated for impaired	818	510	818	510
Ending balance: collectively evaluated for impaired	15,310	17,128	15,310	17,128
Total	1,280,784	1,232,270	1,280,784	1,232,270	1,273,661
Ending balance: individually evaluated for impaired	27,862	31,493	27,862	31,493
Ending balance: collectively evaluated for impaired	1,252,922	1,200,777	1,252,922	1,200,777
Consumer Loan [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,139	4,252	4,132	4,244
Charge-offs	(767)	(797)	(1,755)	(1,738)
Recoveries	243	146	337	264
Provision for Loan, Lease, and Other Losses	527	608	1,428	1,439
Ending Balance	4,142	4,209	4,142	4,209
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	4,142	4,209	4,142	4,209
Total	600,580	576,534	600,580	576,534	582,218
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	600,580	576,534	600,580	576,534
Unallocated [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	5,896	5,837	5,926	4,836
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for Loan, Lease, and Other Losses	26	70	(4)	1,071
Ending Balance	5,922	5,907	5,922	5,907
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	$ 5,922	$ 5,907	$ 5,922	$ 5,907