Allowance for Loan Losses - Recorded Investment in Commercial Loan Classes by Internally Assigned Risk Ratings (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 2,821,524	$ 2,593,880
Total percentage of recorded investment in commercial loan	100.00%	100.00%
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	2,690,620	2,430,926
Total percentage of recorded investment in commercial loan	95.40%	93.70%
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	82,184	106,779
Total percentage of recorded investment in commercial loan	2.90%	4.10%
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	48,720	56,175
Total percentage of recorded investment in commercial loan	1.70%	2.20%
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	1,691,649	1,607,756
Total percentage of recorded investment in commercial loan	100.00%	100.00%
Commercial Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	1,624,718	1,519,720
Total percentage of recorded investment in commercial loan	96.00%	94.50%
Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	41,628	57,073
Total percentage of recorded investment in commercial loan	2.50%	3.60%
Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	25,303	30,963
Total percentage of recorded investment in commercial loan	1.50%	1.90%
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	946,366	842,449
Total percentage of recorded investment in commercial loan	100.00%	100.00%
Commercial and Industrial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	903,273	792,029
Total percentage of recorded investment in commercial loan	95.50%	94.00%
Commercial and Industrial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	27,809	34,085
Total percentage of recorded investment in commercial loan	2.90%	4.10%
Commercial and Industrial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	15,284	16,335
Total percentage of recorded investment in commercial loan	1.60%	1.90%
Commercial Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	183,509	143,675
Total percentage of recorded investment in commercial loan	100.00%	100.00%
Commercial Construction [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	162,629	119,177
Total percentage of recorded investment in commercial loan	88.60%	82.90%
Commercial Construction [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	12,747	15,621
Total percentage of recorded investment in commercial loan	6.90%	10.90%
Commercial Construction [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 8,133	$ 8,877
Total percentage of recorded investment in commercial loan	4.50%	6.20%