Allowance for Loan Losses - Recorded Investment in Commercial Loan Classes by Internally Assigned Risk Ratings (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 3,733,828	$ 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	$ 3,551,246	$ 2,780,315
Total percentage of recorded investment in commercial loan	95.10%	96.10%
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 96,892	$ 65,968
Total percentage of recorded investment in commercial loan	2.60%	2.30%
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 85,690	$ 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	$ 2,111,585	$ 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	$ 2,036,905	$ 1,635,132
Total percentage of recorded investment in commercial loan	96.50%	97.20%
Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 32,371	$ 23,597
Total percentage of recorded investment in commercial loan	1.50%	1.40%
Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 42,309	$ 23,507
Total percentage of recorded investment in commercial loan	2.00%	1.40%
Commercial Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 384,328	$ 216,148
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	$ 344,513	$ 196,520
Total percentage of recorded investment in commercial loan	89.60%	90.90%
Commercial Construction [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 18,771	$ 12,014
Total percentage of recorded investment in commercial loan	4.90%	5.60%
Commercial Construction [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 21,044	$ 7,614
Total percentage of recorded investment in commercial loan	5.50%	3.50%
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 1,237,915	$ 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	$ 1,169,828	$ 948,663
Total percentage of recorded investment in commercial loan	94.50%	95.40%
Commercial and Industrial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 45,750	$ 30,357
Total percentage of recorded investment in commercial loan	3.70%	3.10%
Commercial and Industrial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in commercial loan	$ 22,337	$ 15,118
Total percentage of recorded investment in commercial loan	1.80%	1.50%