Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments, at cost	$ 37,885	$ 26,661
State tax credits, held for sale	45,625	43,468
State Tax Credits Held For Sale, Fair Value Disclosure	$ 171	400
State Tax Credit Term	10 years
Receivables, Fair Value Disclosure	$ 700
Impaired Financing Receivable, Related Allowance	200
Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
State tax credits, held for sale	45,600
Recurring basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Investments	0
State Tax Credits Held For Sale, Fair Value Disclosure	171	400
Portion at Other than Fair Value, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Investments, Fair Value Disclosure	37,700
State Tax Credits Held For Sale, Fair Value Disclosure	45,500
Estimated fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
State Tax Credits Held For Sale, Fair Value Disclosure	48,622	44,271
Estimated fair value | Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Investments	144
State Tax Credits Held For Sale, Fair Value Disclosure	$ 171	$ 400