Long-term Debt - Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Credit Agreement [Member]
Debt Instrument [Line Items]
Debt	$ 0	$ 10,810
Senior Notes [Member]
Debt Instrument [Line Items]
Debt	300,000	300,000
Revolving Credit Facility [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Debt	0	10,810
Fair Value, Inputs, Level 2 [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Fair Value	288,000	300,750
Fair Value, Inputs, Level 2 [Member] | Revolving Credit Facility [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Long-term Debt, Fair Value	$ 0	$ 10,490