Loans, Leases and Allowance for Credit Losses - Schedule of Held for Investment Loan Portfolio Composition of Loans, Leases and Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Interest Rate and Credit Marks on Acquired Loans	$ 47,400	$ 27,100
Trade and Loans Receivables Held-for-sale, Net, Not Part of Disposal Group	24,356	0
Net deferred loan fees and costs	(16,900)	(12,500)
Loans, net of deferred loan fees and costs	10,763,939	8,398,265	$ 7,929,500
Allowance for credit losses	(117,072)	(110,216)	(109,200)
Loans, net	10,646,867	8,288,049	$ 7,820,300
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	4,799,423	3,326,708
Commercial Real Estate Non Owner Occupied Multi Family [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	2,210,642	2,052,566
Owner occupied [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	2,123,882	1,732,888
Allowance for credit losses	(11,624)	(12,454)
Construction and land development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	1,122,094	748,053
Allowance for credit losses	(20,285)	(18,558)
Residential real estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	320,679	299,402
Allowance for credit losses	(5,310)	(7,456)
Leases [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	160,593	205,639
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	26,626	33,009
Allowance for credit losses	$ (524)	$ (853)