Commitments and Contingencies - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies [Line Items]
Letters of credit, outstanding	$ 5.3	$ 5.9
Minimum
Commitments and Contingencies [Line Items]
Operating Lease Term	1 year
Maximum
Commitments and Contingencies [Line Items]
Operating Lease Term	10 years