Long-Term Debt (Details) - USD ($)	1 Months Ended
	May 31, 2017	Jun. 30, 2018	Dec. 31, 2017	May 03, 2017	Dec. 31, 2015
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, amount outstanding		$ 36,800,000	$ 36,900,000
2015 Credit Agreement [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity				$ 100,000,000.0	$ 250,000,000.0
Covenant, minimum unrestricted cash	$ 200,000,000.0
Covenant, cash and cash equivalents held in accounts subject to control agreements	$ 100,000,000.0
Credit facility, amount outstanding		$ 0
2015 Credit Agreement [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity				50,000,000.0	50,000,000.0
2015 Credit Agreement [Member] | Swing Line Loan [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity				$ 25,000,000.0	$ 25,000,000.0
2015 Credit Agreement [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Stated interest rate				1.50%
2015 Credit Agreement [Member] | Minimum [Member] | Line of Credit [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
2015 Credit Agreement [Member] | Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Stated interest rate				2.00%
2015 Credit Agreement [Member] | Maximum [Member] | Line of Credit [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%