Leases - Finance Leases (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Leases
Finance Leases, net of accumulated amortization	$ 1,500,000	$ 595,000,000,000
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Property, Plant and Equipment, Net	Property, Plant and Equipment, Net
Finance leases, amortization expense included in operating expenses	$ 64,000	$ 46,000
Obligations under finance leases	$ 0