Allowance for Loan Losses - Recorded Investment in Commercial Loan Classes by Internally Assigned Risk Ratings (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 3,639,300	$ 2,892,522
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	$ 3,436,177	$ 2,780,315
Total percentage of recorded investment in commercial loan	94.40%	96.10%
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 119,855	$ 65,968
Total percentage of recorded investment in commercial loan	3.30%	2.30%
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 83,268	$ 46,239
Total percentage of recorded investment in commercial loan	2.30%	1.60%
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 2,054,935	$ 1,682,236
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	$ 1,976,311	$ 1,635,132
Total percentage of recorded investment in commercial loan	96.20%	97.20%
Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 33,328	$ 23,597
Total percentage of recorded investment in commercial loan	1.60%	1.40%
Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 45,296	$ 23,507
Total percentage of recorded investment in commercial loan	2.20%	1.40%
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 1,239,382	$ 994,138
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	$ 1,158,898	$ 948,663
Total percentage of recorded investment in commercial loan	93.50%	95.40%
Commercial and Industrial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 60,251	$ 30,357
Total percentage of recorded investment in commercial loan	4.90%	3.10%
Commercial and Industrial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 20,233	$ 15,118
Total percentage of recorded investment in commercial loan	1.60%	1.50%
Construction Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 344,983	$ 216,148
Total percentage of recorded investment in commercial loan	100.00%	100.00%
Construction Loans [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 300,968	$ 196,520
Total percentage of recorded investment in commercial loan	87.30%	90.90%
Construction Loans [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 26,276	$ 12,014
Total percentage of recorded investment in commercial loan	7.60%	5.60%
Construction Loans [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 17,739	$ 7,614
Total percentage of recorded investment in commercial loan	5.10%	3.50%