Equity - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Aggregate Intrinsic Value
Outstanding intrinsic value	$ 23.4
Exercisable intrinsic value	$ 23.4
Stock options
Share-Based Compensation [Line Items]
Vesting period	3 years
Shares
Outstanding at the beginning of the period (in shares)	0.7
Exercised (in shares)	(0.1)
Outstanding at the ending of the period (in shares)	0.6	0.7
Exercisable at the end of the period (in shares)	0.6
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 22.71
Exercised (in dollars per share)	22.72
Outstanding at the ending of the period (in dollars per share)	22.71	$ 22.71
Exercisable at the end of the period (in dollars per share)	$ 22.71
Weighted Average Remaining Contractual Life (in years)
Outstanding at the end of the period (in years)	2 years 9 months 18 days
Exercisable at the end of the period (in years)	2 years 9 months 18 days
Aggregate Intrinsic Value
Exercised intrinsic value	$ 3.3	$ 5.8	$ 15.4