Long-Term Debt - Narrative (Details) - USD ($)			3 Months Ended	9 Months Ended		12 Months Ended
	Jul. 31, 2018	Mar. 09, 2018	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016	Jan. 31, 2018	Jan. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Repayment of revolving credit facility			$ 480,000,000	$ 725,000,000	$ 45,000,000
Unamortized debt issuance expense			2,900,000	2,900,000					$ 1,400,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 800,000,000					$ 530,000,000	$ 350,000,000
Incurred fees and expenses		$ 2,000,000.0
Additional borrowing capacity available			$ 350,000,000	$ 350,000,000
Commitment fee				0.20%
Revolving Credit Facility | Federal Funds Effective Swap Rate
Line of Credit Facility [Line Items]
Basis spread				0.50%
Revolving Credit Facility | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread				1.00%
Term Loan Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 500,000,000
Commitment fee						0.20%
Unamortized debt issuance expense	$ 1,100,000
Term Loan Credit Facility | Overnight Bank Funding Rate
Line of Credit Facility [Line Items]
Basis spread				0.50%
Minimum | Term Loan Credit Facility | Base Rate
Line of Credit Facility [Line Items]
Basis spread	0.00%
Minimum | Term Loan Credit Facility | Eurodollar
Line of Credit Facility [Line Items]
Basis spread	1.00%
Maximum | Term Loan Credit Facility | Base Rate
Line of Credit Facility [Line Items]
Basis spread	0.50%
Maximum | Term Loan Credit Facility | Eurodollar
Line of Credit Facility [Line Items]
Basis spread	1.50%