Debt - Senior Unsecured Notes (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2017
Long-term Debt, Total	$ 775,200
Senior Unsecured Debt [Member]
Total Principal	385,000	$ 385,000
Unamortized debt issuance costs	(883)	(936)
Long-term Debt, Total	384,117	384,064
Senior Unsecured Debt [Member] | 2025 Senior Unsecured Notes [Member]
Total Principal	50,000	50,000
Senior Unsecured Debt [Member] | 2027 Senior Unsecured Notes [Member]
Total Principal	50,000	50,000
Senior Unsecured Debt [Member] | 2028 Senior Unsecured Notes [Member]
Total Principal	60,000	60,000
Senior Unsecured Debt [Member] | 2029 Senior Unsecured Notes [Member]
Total Principal	100,000	100,000
Senior Unsecured Debt [Member] | 2030 Senior Unsecured Notes [Member]
Total Principal	$ 125,000	$ 125,000