Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Pension and other postretirement benefits
Aggregate contributions to qualified and non-qualified pension trusts and payments of pension benefits for unfunded pension plans			$ 5.6	$ 1.3	$ 2.8
Pension Benefits
Pension and other postretirement benefits
Service cost	$ 1.2	$ 1.4	3.6	4.2
Interest cost	4.0	3.6	12.0	10.2
Expected return on plan assets	(4.7)	(5.0)	(14.2)	(14.1)
Recognized net actuarial loss	1.6	1.6	4.9	4.7
Amortization of prior service benefit	0.1	0.1	0.2	0.2
Net periodic benefit cost	2.2	1.7	6.5	5.2
Expected 2016 aggregate contributions to qualified and non-qualified pension trusts and payments of pension benefits for unfunded pension plans			10.0
Postretirement Benefits Other than Pensions
Pension and other postretirement benefits
Service cost	0.3	0.4	0.9	1.1
Interest cost	0.4	0.4	1.2	1.2
Recognized net actuarial loss	0.1	0.0	0.2	0.1
Amortization of prior service benefit	(0.1)	(0.1)	(0.2)	(0.2)
Net periodic benefit cost	$ 0.7	$ 0.7	$ 2.1	$ 2.2