Fair Value Measurements - Financial Instruments Measured at Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Assets
Securities available for sale	$ 448,049	$ 434,587
State tax credits held for sale	14,985	16,491
Liabilities
State tax credits, held for sale	45,529	48,457
Tax credit stream, term	10 years
Years of tax credits generated	10 years
Obligations of U.S. Government sponsored enterprises
Assets
Securities available for sale	92,971	93,530
Obligations of states and political subdivisions
Assets
Securities available for sale	49,933	48,943
Residential mortgage-backed securities
Assets
Securities available for sale	305,145	292,114
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
State tax credits held for sale	0	0
Significant Other Observable Inputs (Level 2)
Assets
State tax credits held for sale	0	0
Significant Unobservable Inputs (Level 3)
Assets
State tax credits held for sale	34,849	35,668
Significant Unobservable Inputs (Level 3) | Auction Rate Securities
Liabilities
Number of securities	3
Total Fair Value
Assets
State tax credits held for sale	34,849	35,668
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Securities available for sale	0
State tax credits held for sale	0
Derivative financial instruments	0
Total assets	0
Liabilities
Derivative financial instruments	0
Total liabilities	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Obligations of U.S. Government sponsored enterprises
Assets
Securities available for sale	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Obligations of states and political subdivisions
Assets
Securities available for sale	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Residential mortgage-backed securities
Assets
Securities available for sale	0
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Securities available for sale	444,998
State tax credits held for sale	0
Derivative financial instruments	1,186
Total assets	446,184
Liabilities
Derivative financial instruments	1,184
Total liabilities	1,184
Recurring basis | Significant Other Observable Inputs (Level 2) | Obligations of U.S. Government sponsored enterprises
Assets
Securities available for sale	92,971
Recurring basis | Significant Other Observable Inputs (Level 2) | Obligations of states and political subdivisions
Assets
Securities available for sale	46,882
Recurring basis | Significant Other Observable Inputs (Level 2) | Residential mortgage-backed securities
Assets
Securities available for sale	305,145
Recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Securities available for sale	3,051
State tax credits held for sale	14,985
Derivative financial instruments	0
Total assets	18,036
Liabilities
Derivative financial instruments	0
Total liabilities	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Obligations of U.S. Government sponsored enterprises
Assets
Securities available for sale	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Obligations of states and political subdivisions
Assets
Securities available for sale	3,051
Recurring basis | Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities
Assets
Securities available for sale	0
Recurring basis | Total Fair Value
Assets
Securities available for sale	448,049
State tax credits held for sale	14,985
Derivative financial instruments	1,186
Total assets	464,220
Liabilities
Derivative financial instruments	1,184
Total liabilities	1,184
Recurring basis | Total Fair Value | Obligations of U.S. Government sponsored enterprises
Assets
Securities available for sale	92,971
Recurring basis | Total Fair Value | Obligations of states and political subdivisions
Assets
Securities available for sale	49,933
Recurring basis | Total Fair Value | Residential mortgage-backed securities
Assets
Securities available for sale	305,145
Portion at Other than Fair Value, Fair Value Disclosure
Assets
State tax credits held for sale	$ 30,500
LIBOR Swap Curve
Liabilities
Discount rate, basis point spread	205