Stock-Based Compensation Plans - Allocated Share-based Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total compensation expense	$ 2.3	$ 4.3	$ 8.5	$ 10.5
2013 Grants
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total compensation expense	0.0	0.2	0.1	0.6
2014 Grants
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total compensation expense	0.3	0.7	0.9	2.0
2015 Grants
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total compensation expense	1.2	2.0	3.1	3.9
2016 Grants
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total compensation expense	0.8	0.0	4.1	0.0
Founders
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total compensation expense	$ 0.0	$ 1.4	$ 0.3	$ 4.0