Nature of Business and Accounting Policies - Foreign Currency Gain (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Derivative term (less than)	1 month
Foreign Currency Translation
Net unrealized losses related to foreign currency translation	$ (21.0)	$ (7.9)	$ (2.1)
Net foreign currency transaction gain (loss)	$ 5.5	$ 4.0	$ (6.8)