Long-Term Debt (Details) - USD ($)	May 03, 2017	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Line of Credit [Member] | 2015 Credit Agreement [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity	$ 100,000,000.0		$ 250,000,000.0	$ 250,000,000.0
Covenant compliance, unrestricted cash liquidity requirement	200,000,000.0
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, amount outstanding		$ 38,000,000	$ 38,000,000
Letter of Credit [Member] | 2015 Credit Agreement [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity	50,000,000.0			50,000,000.0
Credit facility, amount outstanding		$ 38,300,000
Swing Line Loan [Member] | 2015 Credit Agreement [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity	$ 25,000,000.0			$ 25,000,000.0
Minimum [Member] | Line of Credit [Member] | 2015 Credit Agreement [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
Minimum [Member] | Line of Credit [Member] | 2015 Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.50%
Maximum [Member] | Line of Credit [Member] | 2015 Credit Agreement [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
Maximum [Member] | Line of Credit [Member] | 2015 Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.00%
Silicon Valley Bank [Member] | Line of Credit [Member] | 2015 Credit Agreement [Member]
Line of Credit Facility [Line Items]
Covenant compliance, unrestricted cash liquidity requirement	$ 100,000,000.0