Fair Value Measurements - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) security	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) security	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
State tax credits held for sale, including $9,965 and $11,689 carried at fair value, respectively	$ 42,062		$ 42,062		$ 38,309
State tax credits, held for sale, carried at fair value	9,965		$ 9,965		11,689
State tax credit term			10 years
Years of tax credits generated			10 years
Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
State tax credits held for sale, including $9,965 and $11,689 carried at fair value, respectively	42,100		$ 42,100
Recurring basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
State tax credits, held for sale, carried at fair value	$ 9,965		$ 9,965		11,689
Auction Rate Securities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of available-for-sale securities | security	3		3
Portion at Other than Fair Value, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
State tax credits, held for sale, carried at fair value	$ 32,100		$ 32,100
Estimated fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
State tax credits, held for sale, carried at fair value	46,549		46,549		42,970
Estimated fair value | Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
State tax credits, held for sale, carried at fair value	9,965		9,965		$ 11,689
State tax credits held for sale, at fair value | Recurring basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized gains relating to assets still held at the reporting date	$ (36)	$ (130)	$ (310)	$ 204
LIBOR | LIBOR Swap Curve [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate basis spread on variable rate	205.00%		205.00%
LIBOR Swap Curve [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate basis point spread			205