Fair Value Information - Estimated Fair Value of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Assets
Total cash and cash equivalents	$ 135,278	$ 98,431	$ 276,318	$ 345,343
Interest-earning balances due from depository institutions	70,000	70,000
FHLB stock	50,981	56,651
Investment securities available-for-sale	2,390,673	2,449,387
Investment securities held-to-maturity	1,975	2,050
Total loans, net of allowance for credit losses	3,097,296	3,159,872
Accrued interest receivable	22,985	22,355
Deposits:
Noninterest-bearing	2,366,719	2,420,993
Interest-bearing	2,319,442	2,352,994
Junior subordinated debentures	46,393	67,012
Accrued interest payable	1,306	1,493
Level 1 [Member]
Assets
Total cash and cash equivalents	135,278	98,431
Deposits:
Noninterest-bearing	2,366,719	2,420,993
Level 2 [Member]
Assets
Interest-earning balances due from depository institutions	70,000	70,000
FHLB stock	50,981	56,651
Investment securities available-for-sale	2,390,673	2,449,387
Accrued interest receivable	22,985	22,355
Swaps	21,358	23,966
Deposits:
Interest-bearing	2,320,514	2,354,126
Borrowings	727,065	727,512
Junior subordinated debentures	46,649	67,415
Accrued interest payable	1,306	1,493
Swaps	21,358	23,966
Level 3 [Member]
Assets
Investment securities held-to-maturity	2,425	2,515
Total loans, net of allowance for credit losses	3,394,235	3,503,332
Carrying Value [Member]
Assets
Total cash and cash equivalents	135,278	98,431
Interest-earning balances due from depository institutions	70,000	70,000
FHLB stock	50,981	56,651
Investment securities available-for-sale	2,390,673	2,449,387
Investment securities held-to-maturity	1,975	2,050
Total loans, net of allowance for credit losses	3,275,990	3,355,087
Accrued interest receivable	22,985	22,355
Swaps	21,358	23,966
Deposits:
Noninterest-bearing	2,366,719	2,420,993
Interest-bearing	2,319,442	2,352,994
Borrowings	699,117	698,178
Junior subordinated debentures	46,393	67,012
Accrued interest payable	1,306	1,493
Swaps	21,358	23,966
Estimated Fair Value [Member]
Assets
Total cash and cash equivalents	135,278	98,431
Interest-earning balances due from depository institutions	70,000	70,000
FHLB stock	50,981	56,651
Investment securities available-for-sale	2,390,673	2,449,387
Investment securities held-to-maturity	2,425	2,515
Total loans, net of allowance for credit losses	3,394,235	3,503,332
Accrued interest receivable	22,985	22,355
Swaps	21,358	23,966
Deposits:
Noninterest-bearing	2,366,719	2,420,993
Interest-bearing	2,320,514	2,354,126
Borrowings	727,065	727,512
Junior subordinated debentures	46,649	67,415
Accrued interest payable	1,306	1,493
Swaps	$ 21,358	$ 23,966