Debt - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of interest expense
Contractual coupon interest	$ 2,154	$ 1,984	$ 6,201	$ 5,694
Amortization of discounts and deferred financing costs	308	278	896	842
Total interest expense	$ 2,462	$ 2,262	$ 7,097	$ 6,536