Debt - Notes (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Roll Forward]
Amortization of debt issuance costs	$ 155	$ 168
Senior Notes
Debt Instrument [Roll Forward]
2033 Senior Notes, beginning balance	31,850
Discount, beginning balance	(2,565)
Debt issuance cost, beginning balance	(125)
Total, beginning balance	29,160
Amortization of debt discount	1,686
Amortization of debt issuance costs	111
Amortization of debt discount and debt issuance costs	1,797
2033 Senior Notes, ending balance	31,850
Discount, ending balance	(879)
Debt issuance cost, ending balance	(14)
Total, ending balance	$ 30,957