Debt - Notes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Debt Instrument [Roll Forward]
Amortization of debt issuance costs	$ 43	$ 56
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	547
Amortization of debt issuance costs	37
Amortization of debt discount and debt issuance costs	584
2033 Senior Notes, ending balance	31,850
Discount, ending balance	(2,018)
Debt issuance cost, ending balance	(88)
Total, ending balance	$ 29,744