Share-Based Compensation Plans - Schedule of Compensation Costs by Performance Incentive Plan (Details) - Performance Shares - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
2013 Executive Performance Incentive Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Capitalized compensation cost	$ 0.0	$ 0.0	$ 4.4
2014 Executive Performance Incentive Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Capitalized compensation cost	0.0	4.2	4.9
2015 Executive Performance Incentive Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Capitalized compensation cost	2.2	4.9	4.9
2016 Executive Performance Incentive Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Capitalized compensation cost	4.4	3.3	0.0
2017 Executive Performance Incentive Program, Liability
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Capitalized compensation cost	$ 1.7	$ 0.0	$ 0.0