Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Other Commitments [Line Items]
Letters of credit outstanding, amount	$ 13.4	$ 16.4
Purchase commitment	$ 265.6
Letter of Credit | Minimum
Other Commitments [Line Items]
Letter of credit, fee percentage	2.50%
Letter of Credit | Maximum
Other Commitments [Line Items]
Letter of credit, fee percentage	3.25%