Carrying Amount and Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Carrying Amount
Financial assets
Cash and cash equivalents	$ 183,466	$ 442,409
Investment securities, available-for-sale	2,544,093	2,387,428
Investment securities, held-to-maturity	570,285	520,997
Loans held for sale	54,132	46,726
Loans receivable, net of ALLL	4,557,813	4,358,342
Accrued interest receivable	43,260	40,587
Non-marketable equity securities	54,277	52,868
Total financial assets	8,007,326	7,849,357
Financial liabilities
Deposits	6,458,792	6,345,212
FHLB advances	298,148	296,944
Repurchase agreements and other borrowed funds	432,355	404,418
Subordinated debentures	125,741	125,705
Accrued interest payable	3,893	4,155
Interest rate swaps	21,410	16,668
Total financial liabilities	7,340,339	7,193,102
Estimated Fair Value | Level 1
Financial assets
Cash and cash equivalents	183,466	442,409
Investment securities, available-for-sale	0	0
Investment securities, held-to-maturity	0	0
Loans held for sale	54,132	46,726
Loans receivable, net of ALLL	0	0
Accrued interest receivable	43,260	40,587
Non-marketable equity securities	0	0
Total financial assets	280,858	529,722
Financial liabilities
Deposits	5,082,499	4,928,771
FHLB advances	0	0
Repurchase agreements and other borrowed funds	0	0
Subordinated debentures	0	0
Accrued interest payable	3,893	4,155
Interest rate swaps	0	0
Total financial liabilities	5,086,392	4,932,926
Estimated Fair Value | Level 2
Financial assets
Cash and cash equivalents	0	0
Investment securities, available-for-sale	2,544,093	2,387,428
Investment securities, held-to-maturity	597,127	550,946
Loans held for sale	0	0
Loans receivable, net of ALLL	4,476,848	4,288,417
Accrued interest receivable	0	0
Non-marketable equity securities	54,277	52,868
Total financial assets	7,672,345	7,279,659
Financial liabilities
Deposits	1,380,870	1,421,234
FHLB advances	314,837	312,363
Repurchase agreements and other borrowed funds	432,355	404,418
Subordinated debentures	77,353	76,711
Accrued interest payable	0	0
Interest rate swaps	21,410	16,668
Total financial liabilities	2,226,825	2,231,394
Estimated Fair Value | Level 3
Financial assets
Cash and cash equivalents	0	0
Investment securities, available-for-sale	0	0
Investment securities, held-to-maturity	0	0
Loans held for sale	0	0
Loans receivable, net of ALLL	152,222	149,769
Accrued interest receivable	0	0
Non-marketable equity securities	0	0
Total financial assets	152,222	149,769
Financial liabilities
Deposits	0	0
FHLB advances	0	0
Repurchase agreements and other borrowed funds	0	0
Subordinated debentures	0	0
Accrued interest payable	0	0
Interest rate swaps	0	0
Total financial liabilities	$ 0	$ 0