Commitments and Contingencies - Leases (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Number of leases that require rent to be paid based on contingent events | item	0
Rent expense | $	$ 30.2	$ 27.4	$ 25.9