Debt Narrative (Details) - USD ($)			1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 23, 2018	Dec. 01, 2016	Jan. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Line of Credit Facility [Line Items]
Borrowings				$ 494,000,000		$ 494,000,000
Term B Loan Facility
Line of Credit Facility [Line Items]
Repayment of debt					$ 1,500,000		$ 3,000,000
Term B Loan Facility | Line of Credit
Line of Credit Facility [Line Items]
Borrowing capacity	$ 494,000,000.0
Debt instrument, maturity date		Nov. 30, 2023
Borrowings		$ 600,000,000		$ 494,000,000		$ 494,000,000
Voluntary prepayment against the term loan	$ 100,000,000		$ 100,000,000
Debt instrument, prepayment premium	1.00%
Interest rate				5.00%		5.00%
Unamortized debt issuance costs				$ 13,000,000		$ 13,000,000
Interest expense				6,200,000	7,000,000	12,500,000	13,500,000
Amortization of debt issuance costs				$ 600,000	$ 600,000	$ 1,300,000	$ 1,200,000
Term B Loan Facility | Line of Credit | Eurodollar
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.50%
Term B Loan Facility | Line of Credit | Base Rate
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	1.50%
Term B Loan Facility | Line of Credit | Royal Bank of Canada
Line of Credit Facility [Line Items]
Borrowing capacity		$ 600,000,000
Loan facility, term		7 years