Fair Value Measurement - Accumulated Derivative Instruments (Loss) Gain (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cumulative Changes in Derivative Net Gain (Loss) [Roll Forward]
Balance at beginning of period	$ 7.5	$ (13.5)	$ (12.5)
Reclassification to earnings	(2.1)	15.0	11.7
Current period change in fair value	(5.7)	6.0	(12.7)
Balance at end of period	(0.3)	$ 7.5	$ (13.5)
Expected reclassification of pre-tax losses in the next twelve months from ACOL to earnings	$ 0.2