Disclosures About Fair Value of Financial Instruments - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying Amount	$ 265,588	$ 264,593	$ 383,399	$ 371,023
Cash and cash equivalents, Estimated Fair Value	265,588	264,593
Investment securities, available for sale, Carrying Amount	530,896	560,253
Investment securities, Estimated Fair Value	530,896	560,253
Loans held for sale, Carrying Amount	2,402,708	1,797,064
Loans held for sale, Estimated Fair Value	2,402,848	1,797,458
Loans receivable, net of allowance for loan losses, Carrying Amount	5,979,098	5,417,832
Loans receivable, net of allowance for loan losses, Estimated Fair Value	5,998,001	5,353,903
FHLB, Federal Reserve Bank and other restricted stock, Carrying Amount	71,621	90,841
FHLB, Federal Reserve Bank and other restricted stock, Estimated Fair Value	71,621	90,841
Derivative assets	18,789	9,295
Deposits, Carrying Amount	7,388,970	5,909,501	$ 5,785,194
Deposits, Estimated Fair Value	7,398,218	5,911,754
Federal funds purchased, Carrying Amount	52,000	70,000
Federal funds purchased, Estimated Fair Value	52,000	70,000
FHLB advances, Carrying Amount	1,036,700	1,625,300
FHLB advances, Estimated Fair Value	1,037,706	1,625,468
Other borrowings, Carrying Amount	86,957	86,457
Other borrowings, Estimated Fair Value	88,967	93,804
Subordinated debt, Carrying Amount	108,758	108,685
Subordinated debt, Estimated Fair Value	111,650	110,825
Derivative liabilities	25,466	13,932
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Estimated Fair Value	265,588	264,593
Investment securities, Estimated Fair Value	16,736	19,212
Loans held for sale, Estimated Fair Value	0	0
Loans receivable, net of allowance for loan losses, Estimated Fair Value	0	0
FHLB, Federal Reserve Bank and other restricted stock, Estimated Fair Value	0	0
Derivative assets	0	0
Deposits, Estimated Fair Value	4,459,937	3,561,905
Federal funds purchased, Estimated Fair Value	52,000	70,000
FHLB advances, Estimated Fair Value	851,700	1,365,300
Other borrowings, Estimated Fair Value	63,655	68,867
Subordinated debt, Estimated Fair Value	0	0
Derivative liabilities	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Estimated Fair Value	0	0
Investment securities, Estimated Fair Value	514,160	541,041
Loans held for sale, Estimated Fair Value	2,377,445	1,757,807
Loans receivable, net of allowance for loan losses, Estimated Fair Value	0	0
FHLB, Federal Reserve Bank and other restricted stock, Estimated Fair Value	71,621	90,841
Derivative assets	18,704	9,250
Deposits, Estimated Fair Value	2,938,281	2,349,849
Federal funds purchased, Estimated Fair Value	0	0
FHLB advances, Estimated Fair Value	186,006	260,168
Other borrowings, Estimated Fair Value	25,312	24,937
Subordinated debt, Estimated Fair Value	111,650	110,825
Derivative liabilities	25,466	13,932
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Estimated Fair Value	0	0
Investment securities, Estimated Fair Value	0	0
Loans held for sale, Estimated Fair Value	25,403	39,651
Loans receivable, net of allowance for loan losses, Estimated Fair Value	5,998,001	5,353,903
FHLB, Federal Reserve Bank and other restricted stock, Estimated Fair Value	0	0
Derivative assets	85	45
Deposits, Estimated Fair Value	0	0
Federal funds purchased, Estimated Fair Value	0	0
FHLB advances, Estimated Fair Value	0	0
Other borrowings, Estimated Fair Value	0	0
Subordinated debt, Estimated Fair Value	0	0
Derivative liabilities	$ 0	$ 0