Commitments and Contingencies - Operating Leases Future Minimum Payments (Details) $ in Thousands	Mar. 31, 2019 $ / €	Mar. 31, 2019 USD ($)	Mar. 31, 2019 $ / £	Dec. 31, 2018 $ / €	Dec. 31, 2018 USD ($)	Dec. 31, 2018 $ / £	Dec. 31, 2018 £ / $	Dec. 31, 2018 € / $
Operating Lease Liabilities, Payments Due [Abstract]
2019 (remainder)		$ 1,008
2020		1,344
2021		1,344
2022		1,344
2023		1,344
2024		1,348
Thereafter		38,382
Total minimum lease payments		46,114
Less: Effects of discounting		(22,397)
Total present value of lease payments		$ 23,717
Foreign currency exchange rate (gbp, eur per usd)	1.12		1.30	1.14		1.27	1.27	1.14
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019					$ 1,371
2020					1,371
2021					1,371
2022					1,371
2023					1,371
Thereafter					40,519
Total minimum lease payments					47,374
Less: Effects of discounting					(23,370)
Total present value of lease payments					$ 24,004