Long-term Debt - Debt Issuance Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Debt issuance costs	$ (7,730)	$ (4,720)
Amortization of Debt Issuance Costs	$ 1,320	$ 1,370	$ 1,710