Assets and Liabilities Measured at Fair Value - Fair values for other financial instruments that are not measured at fair value on recurring basis (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Level 1
Assets:
Securities held to maturity
Loans, net
Mortgage loans held for sale
Liabilities:
Deposits
Federal Home Loan Bank advances
Long-term debt
Level 2
Assets:
Securities held to maturity	247,087	261,131	318,490
Loans, net
Mortgage loans held for sale	18,803	29,693	25,288
Liabilities:
Deposits	6,034,103	5,988,743	5,986,925
Federal Home Loan Bank advances	125	40,125	217,033
Long-term debt
Level 3
Assets:
Securities held to maturity
Loans, net	3,980,932	3,957,669	3,825,482
Mortgage loans held for sale
Liabilities:
Deposits
Federal Home Loan Bank advances
Long-term debt	123,402	118,626	113,891
Total
Assets:
Securities held to maturity	247,087	261,131	318,490
Loans, net	3,980,932	3,957,669	3,825,482
Mortgage loans held for sale
Liabilities:
Deposits	6,034,103	5,988,743	5,986,925
Federal Home Loan Bank advances	125	40,125	217,033
Long-term debt	123,402	118,626	113,891
Carrying Amount
Assets:
Securities held to maturity	231,087	244,184	303,636
Loans, net	4,087,807	4,067,871	4,013,965
Mortgage loans held for sale	18,290	28,821	24,809
Liabilities:
Deposits	6,025,852	5,952,140	6,000,539
Federal Home Loan Bank advances	125	40,125	215,125
Long-term debt	$ 124,825	$ 124,805	$ 120,245