Long-Term Debt - Borrowings (Details) - USD ($) $ in Millions						1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Oct. 06, 2017	Mar. 17, 2017	Nov. 08, 2016	Sep. 21, 2016	Nov. 20, 2013	Jun. 30, 2015	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Long-Term Debt
Unamortized discount							$ 6.0				$ 6.3
Secured Debt | Existing Senior Secured Credit Facility
Long-Term Debt
Maximum borrowing capacity		$ 740.0
Maximum borrowing capacity, increase in capacity		$ 95.0
Secured Debt | Line of Credit | Prior Senior Secured Credit Facility
Long-Term Debt
Maximum borrowing capacity					$ 545.0
Secured Debt | Line of Credit | Prior Senior Secured Credit Facility, Term Loan Facility
Long-Term Debt
Maximum borrowing capacity				$ 828.2	525.0
Unamortized discount					$ 5.3
Original issue discount (as a percentage)					1.00%
Maximum borrowing capacity, increase in capacity				135.0		$ 95.0			$ 55.0	$ 117.0
Original issue discount from increase in capacity				$ 16.5
Principal payment frequency					quarterly
Secured Debt | Line of Credit | Existing Senior Secured Credit Facility
Long-Term Debt
Maximum borrowing capacity			$ 680.0
Secured Debt | Line of Credit | Existing Senior Secured Credit Facility, Term Loan Facility
Long-Term Debt
Maximum borrowing capacity	$ 939.5		$ 645.0
Term			7 years
Maximum borrowing capacity, increase in capacity	205.0
Quarterly amortization payment	$ 2.4
Secured Debt | Revolving Credit Facility
Long-Term Debt
Principal payment frequency			quarterly
Quarterly amortization payment							$ 2.4	$ 1.9
Secured Debt | Revolving Credit Facility | Prior Senior Secured Credit Facility, Revolving Credit Facility
Long-Term Debt
Maximum borrowing capacity					$ 20.0
Secured Debt | Revolving Credit Facility | Existing Senior Secured Credit Facility, Revolving Credit Facility
Long-Term Debt
Maximum borrowing capacity			$ 35.0
Term			5 years