Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments And Contingencies [Line Items]
Rent expense	$ 17.7	$ 17.9	$ 18.5
Minimum
Commitments And Contingencies [Line Items]
LOC contract period requirement		1 year
Operating leases, term of contract		1 month
Maximum
Commitments And Contingencies [Line Items]
LOC contract period requirement		3 years
Operating leases, term of contract		5 years