Debt (Debt Issuance Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument
Amortization of debt issuance costs	$ 2,014	$ 1,536	$ 1,548
Interest paid	17,900	14,300	11,400
Credit Agreement 2015 | Revolving Credit Facility
Debt Instrument
Debt issuance cost			2,300
Amortization of debt issuance costs	$ 1,000	$ 1,000	$ 1,000