Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Document Period End Date	Dec. 31, 2016
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 9.0	$ 3.7	$ 2.9
Interest cost	27.4	23.8	23.0
Expected return on plan assets	(30.5)	(27.0)	(25.8)
Amortization of prior service cost	0.0	0.0	(0.2)
Recognized net actuarial loss	5.5	6.6	3.0
Curtailment loss	(4.6)	0.0	0.0
Net periodic pension benefit cost	6.8	7.1	2.9
Other Benefits
Components of net periodic benefit cost
Service cost	0.0	0.0	0.0
Interest cost	0.5	0.6	0.6
Expected return on plan assets	0.0	0.0	0.0
Amortization of prior service cost	0.0	(0.2)	(0.2)
Recognized net actuarial loss	0.2	0.3	0.2
Curtailment loss	0.0	0.0	0.0
Net periodic pension benefit cost	$ 0.7	$ 0.7	$ 0.6