Fair Value Measurements - Financial Instruments Measured at Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Assets
Securities available for sale	$ 456,584	$ 434,587
State tax credits held for sale	15,131	16,491
Liabilities
State tax credits, held for sale	45,631	48,457
Tax credit stream, term	10 years
Years of tax credits generated	10 years
LIBOR Swap Curve
Liabilities
Discount rate, basis point spread (percent)	0.0205
Portion Measured as Cost
Assets
State tax credits held for sale	30,500
Obligations of U.S. Government-sponsored enterprises
Assets
Securities available for sale	92,272	93,530
Obligations of states and political subdivisions
Assets
Securities available for sale	49,941	48,943
Agency mortgage-backed securities
Assets
Securities available for sale	314,371	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	35,906	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	35,906	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) | Agency mortgage-backed securities
Assets
Securities available for sale	0
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Securities available for sale	453,530
State tax credits held for sale	0
Derivative financial instruments	951
Total assets	454,481
Liabilities
Derivative financial instruments	949
Total liabilities	949
Recurring basis | Significant Other Observable Inputs (Level 2) | Obligations of U.S. Government-sponsored enterprises
Assets
Securities available for sale	92,272
Recurring basis | Significant Other Observable Inputs (Level 2) | Obligations of states and political subdivisions
Assets
Securities available for sale	46,887
Recurring basis | Significant Other Observable Inputs (Level 2) | Agency mortgage-backed securities
Assets
Securities available for sale	314,371
Recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Securities available for sale	3,054
State tax credits held for sale	15,131
Derivative financial instruments	0
Total assets	18,185
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	3,054
Recurring basis | Significant Unobservable Inputs (Level 3) | Agency mortgage-backed securities
Assets
Securities available for sale	0
Recurring basis | Total Fair Value
Assets
Securities available for sale	456,584
State tax credits held for sale	15,131
Derivative financial instruments	951
Total assets	472,666
Liabilities
Derivative financial instruments	949
Total liabilities	949
Recurring basis | Total Fair Value | Obligations of U.S. Government-sponsored enterprises
Assets
Securities available for sale	92,272
Recurring basis | Total Fair Value | Obligations of states and political subdivisions
Assets
Securities available for sale	49,941
Recurring basis | Total Fair Value | Agency mortgage-backed securities
Assets
Securities available for sale	$ 314,371