Debt - Debt Covenants (Narrative) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Apr. 28, 2016	Aug. 31, 2015	Aug. 05, 2015
Debt Instrument [Line Items]
Debt, carrying amount	$ 1,028,375,000	$ 1,065,472,000
Term Credit Agreement
Debt Instrument [Line Items]
Debt, carrying amount	350,000,000	350,000,000
Incremental Term Loan Agreement
Debt Instrument [Line Items]
Debt, carrying amount	$ 300,000,000	$ 300,000,000	$ 300,000,000
Term Loan Facility | Term Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity					$ 350,000,000
Debt, carrying amount					$ 350,000,000
Revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 200,000,000