Senior Debt (Details) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Senior Debt (Table Details) [Abstract]
Unamortized Debt Issuance Expense	$ 2,100			$ 2,100		$ 2,100	$ 5,200
Letters of credit, amount outstanding	92,200			92,200		92,200
Line of credit commitment fee				$ 1,100
Line of credit commitment fee percentage				0.50%
Senior credit facilities, additional amount allowed	100,000			$ 100,000		$ 100,000
Debt refinancing charges	$ 0	$ 0	$ 1,900	$ 0	$ 1,936
Debt arrangement and amendment fees			$ 5,300
Financial Covenants Ratios (Table Details) [Abstract]
Required consolidated fixed charge coverage ratio, minimum	1.10			1.10		1.10
Actual consolidated fixed charge coverage ratio	2.03			2.03		2.03
Covenants [Abstract]
Adjusted consolidated EBITDA						$ 92,500
Consolidated fixed charges						$ 45,500
Event of default-3rd party ownership % of our voting stock	35.00%			35.00%		35.00%
Event of default-not satisfied or bonded judgment in excess of	$ 50,000			$ 50,000		$ 50,000
Maximum period allowed for pending judgment to satisfy or bond (in days)	30			30		30
Minimum [Member]
Senior Debt (Table Details) [Abstract]
Line of credit commitment fee percentage				0.30%
Maximum [Member]
Senior Debt (Table Details) [Abstract]
Line of credit commitment fee percentage				0.50%
Eurodollar [Member]
Senior Debt (Table Details) [Abstract]
Description of variable rate basis				Eurodollar rate
Prime Rate [Member]
Senior Debt (Table Details) [Abstract]
Description of variable rate basis				prime
Term Loan [Member]
Debt Instrument [Line Items]
Senior Debt, Maximum Borrowing Capacity	$ 225,000			$ 225,000		$ 225,000
Senior Debt (Table Details) [Abstract]
Term loan, amount outstanding	0			0		0	48,600
Revolving Credit Facility [Member]
Senior Debt (Table Details) [Abstract]
Senior credit facility, maximum amount	350,000			350,000		350,000
Revolving facility, amount outstanding	0			$ 0		0	85,000
Maturity date				Mar. 19, 2019
Covenants [Abstract]
Line of Credit Facility, Remaining Borrowing Capacity	$ 247,300			$ 247,300		$ 247,300
Revolving Credit Facility [Member] | Eurodollar [Member]
Senior Debt (Table Details) [Abstract]
Margin on Eurodollar rate, actual	2.75%			2.75%		2.75%
Revolving Credit Facility [Member] | Eurodollar [Member] | Minimum [Member]
Senior Debt (Table Details) [Abstract]
Basis margin on variable rate				1.50%
Revolving Credit Facility [Member] | Eurodollar [Member] | Maximum [Member]
Senior Debt (Table Details) [Abstract]
Basis margin on variable rate				3.00%
Revolving Credit Facility [Member] | Prime Rate [Member]
Senior Debt (Table Details) [Abstract]
Margin on Prime Rate, actual	1.75%			1.75%		1.75%
Revolving Credit Facility [Member] | Prime Rate [Member] | Minimum [Member]
Senior Debt (Table Details) [Abstract]
Basis margin on variable rate				0.50%
Revolving Credit Facility [Member] | Prime Rate [Member] | Maximum [Member]
Senior Debt (Table Details) [Abstract]
Basis margin on variable rate				2.00%
Line of Credit [Member]
Covenants [Abstract]
Unsecured line of credit, maximum facility	$ 12,500			$ 12,500		$ 12,500
Unsecured line of credit, amount outstanding	$ 0			$ 0		$ 0	$ 5,700