Fair Value Information - Estimated Fair Value of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Total cash and cash equivalents	$ 105,768	$ 94,693	$ 98,431	$ 345,343
Interest-earning balances due from depository institutions	27,118	70,000
FHLB stock	25,338	32,331
Investment securities available-for-sale	3,137,158	2,663,642
Investment securities held-to-maturity	1,528	1,777
Total loans, net of allowance for loan losses	3,757,242	3,470,996
Accrued interest receivable	23,194	22,051
Swaps	10,080	10,846
Deposits:
Noninterest-bearing	2,866,365	2,562,980
Interest-bearing	2,738,293	2,327,651
Junior subordinated debentures	25,774	25,774
Accrued interest payable	1,161	1,111
Swaps	573,707	654,097
Carrying Value [Member]
Assets
Total cash and cash equivalents	105,768	94,693
Interest-earning balances due from depository institutions	27,118	70,000
FHLB stock	25,338	32,331
Investment securities available-for-sale	3,137,158	2,663,642
Investment securities held-to-maturity	1,528	1,777
Loans held-for-sale	0	3,667
Total loans, net of allowance for loan losses	3,757,242	3,470,996
Accrued interest receivable	23,194	22,051
Swaps	10,080	10,846
Deposits:
Noninterest-bearing	2,866,365	2,562,980
Interest-bearing	2,738,293	2,327,651
Borrowings	809,106	911,457
Junior subordinated debentures	25,774	25,774
Accrued interest payable	1,161	1,111
Swaps	10,080	10,846
Estimated Fair Value [Member]
Assets
Total cash and cash equivalents	105,768	94,693
Interest-earning balances due from depository institutions	27,118	70,000
FHLB stock	25,338	32,331
Investment securities available-for-sale	3,137,158	2,663,642
Investment securities held-to-maturity	2,177	2,296
Loans held-for-sale	0	8,897
Total loans, net of allowance for loan losses	3,794,454	3,527,725
Accrued interest receivable	23,194	22,051
Swaps	10,080	10,846
Deposits:
Noninterest-bearing	2,866,365	2,562,980
Interest-bearing	2,739,221	2,328,488
Borrowings	822,607	932,408
Junior subordinated debentures	26,005	25,819
Accrued interest payable	1,161	1,111
Swaps	10,080	10,846
Estimated Fair Value [Member] | Level 1 [Member]
Assets
Total cash and cash equivalents	105,768	94,693
Interest-earning balances due from depository institutions	0	0
FHLB stock	0	0
Investment securities available-for-sale	0	0
Investment securities held-to-maturity	0	0
Loans held-for-sale	0	0
Total loans, net of allowance for loan losses	0	0
Accrued interest receivable	0	0
Swaps	0	0
Deposits:
Noninterest-bearing	2,866,365	2,562,980
Interest-bearing	0	0
Borrowings	0	0
Junior subordinated debentures	0	0
Accrued interest payable	0	0
Swaps	0	0
Estimated Fair Value [Member] | Level 2 [Member]
Assets
Total cash and cash equivalents	0	0
Interest-earning balances due from depository institutions	27,118	70,000
FHLB stock	25,338	32,331
Investment securities available-for-sale	3,137,158	2,663,642
Investment securities held-to-maturity	0	0
Loans held-for-sale	0	0
Total loans, net of allowance for loan losses	0	0
Accrued interest receivable	23,194	22,051
Swaps	10,080	10,846
Deposits:
Noninterest-bearing	0	0
Interest-bearing	2,739,221	2,328,488
Borrowings	822,607	932,408
Junior subordinated debentures	26,005	25,819
Accrued interest payable	1,161	1,111
Swaps	10,080	10,846
Estimated Fair Value [Member] | Level 3 [Member]
Assets
Total cash and cash equivalents	0	0
Interest-earning balances due from depository institutions	0	0
FHLB stock	0	0
Investment securities available-for-sale	0	0
Investment securities held-to-maturity	2,177	2,296
Loans held-for-sale	0	8,897
Total loans, net of allowance for loan losses	3,794,454	3,527,725
Accrued interest receivable	0	0
Swaps	0	0
Deposits:
Noninterest-bearing	0	0
Interest-bearing	0	0
Borrowings	0	0
Junior subordinated debentures	0	0
Accrued interest payable	0	0
Swaps	$ 0	$ 0