Rollforward of Allowance for Loan Losses by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 75,514	$ 69,881	$ 74,751	$ 71,426
Charge-offs	(4,579)	(4,085)	(9,264)	(8,933)
Recoveries	867	851	1,815	2,154
Provision	5,000	5,000	9,500	7,000
Ending Balance	76,802	71,647	76,802	71,647
Ending Balance: individually evaluated for impairment, allowances	3,615	3,210	3,615	3,210	4,252
Ending Balance: collectively evaluated for impairment, allowances	73,187	68,437	73,187	68,437
Ending Balance: loans	6,920,683	6,338,921	6,920,683	6,338,921	6,520,512
Ending Balance: individually evaluated for impairment, loans	26,925	24,822	26,925	24,822
Ending Balance: collectively evaluated for impairment, loans	6,893,758	6,314,099	6,893,758	6,314,099
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	48,363	43,345	48,886	43,390
Charge-offs	(1,476)	(941)	(2,947)	(2,423)
Recoveries	201	141	268	515
Provision	5,345	2,563	6,226	3,626
Ending Balance	52,433	45,108	52,433	45,108
Ending Balance: individually evaluated for impairment, allowances	2,497	2,727	2,497	2,727
Ending Balance: collectively evaluated for impairment, allowances	49,936	42,381	49,936	42,381
Ending Balance: loans	3,658,124	3,443,175	3,658,124	3,443,175
Ending Balance: individually evaluated for impairment, loans	14,517	14,326	14,517	14,326
Ending Balance: collectively evaluated for impairment, loans	3,643,607	3,428,849	3,643,607	3,428,849
Real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	16,091	14,946	15,342	15,506
Charge-offs	(55)	(176)	(181)	(371)
Recoveries	8	7	17	16
Provision	(1,827)	1,519	(961)	1,145
Ending Balance	14,217	16,296	14,217	16,296
Ending Balance: individually evaluated for impairment, allowances	1,118	483	1,118	483
Ending Balance: collectively evaluated for impairment, allowances	13,099	15,813	13,099	15,813
Ending Balance: loans	2,859,563	2,495,294	2,859,563	2,495,294
Ending Balance: individually evaluated for impairment, loans	12,407	10,465	12,407	10,465
Ending Balance: collectively evaluated for impairment, loans	2,847,156	2,484,829	2,847,156	2,484,829
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	10,984	11,529	10,447	12,470
Charge-offs	(3,048)	(2,968)	(6,136)	(6,139)
Recoveries	658	703	1,530	1,623
Provision	1,480	904	4,233	2,214
Ending Balance	10,074	10,168	10,074	10,168
Ending Balance: individually evaluated for impairment, allowances	0		0
Ending Balance: collectively evaluated for impairment, allowances	10,074	10,168	10,074	10,168
Ending Balance: loans	379,203	374,202	379,203	374,202
Ending Balance: individually evaluated for impairment, loans	1	31	1	31
Ending Balance: collectively evaluated for impairment, loans	379,202	374,171	379,202	374,171
Leases [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	76	61	76	60
Charge-offs	0		0
Recoveries	0		0
Provision	2	14	2	15
Ending Balance	78	75	78	75
Ending Balance: individually evaluated for impairment, allowances	0		0
Ending Balance: collectively evaluated for impairment, allowances	78	75	78	75
Ending Balance: loans	23,793	26,250	23,793	26,250	23,981
Ending Balance: individually evaluated for impairment, loans	0		0
Ending Balance: collectively evaluated for impairment, loans	$ 23,793	$ 26,250	$ 23,793	$ 26,250