Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net periodic pension and OPEB costs:
Other pension and postretirement benefits (costs), net	$ 9.9	$ 9.4	$ 19.9	$ 22.7
Contributions paid to defined benefit plans			5.0
Defined Benefit Plans
Net periodic pension and OPEB costs:
Service cost	1.4	1.9	2.8	3.7
Interest cost	40.7	50.8	81.9	102.3
Expected return on plan assets	(58.9)	(71.4)	(118.6)	(142.1)
Amortization of prior service cost (benefit)	0.2	0.1	0.4	0.3
Amortization of net actuarial loss (gain)	2.0	3.6	3.9	4.7
Curtailment and settlement loss (gain)			(0.1)	0.0
Less: expected participant contributions	0.0	(0.1)	0.0	(0.3)
Other pension and postretirement benefits (costs), net	(16.0)	(17.0)	(32.3)	(35.1)
Net periodic pension and OPEB cost	(14.6)	(15.1)	(29.5)	(31.4)
Other Postretirement Benefits
Net periodic pension and OPEB costs:
Service cost	2.3	2.8	4.6	5.4
Interest cost	6.6	7.6	13.2	15.2
Expected return on plan assets	0.0	0.0	0.0	0.1
Amortization of prior service cost (benefit)	(0.1)	0.0	(0.1)	0.0
Amortization of net actuarial loss (gain)	(0.4)	0.0	(0.7)	0.0
Curtailment and settlement loss (gain)			0.0	2.9
Less: expected participant contributions	0.0	0.0	0.0	0.0
Other pension and postretirement benefits (costs), net	6.1	7.6	12.4	12.4
Net periodic pension and OPEB cost	8.4	10.4	17.0	17.8
defined benefit plans and other postretirement benefit plans [Member]
Net periodic pension and OPEB costs:
Service cost	3.7	4.7	7.4	9.1
Interest cost	47.3	58.4	95.1	117.5
Expected return on plan assets	(58.9)	(71.4)	(118.6)	(142.0)
Amortization of prior service cost (benefit)	0.1	0.1	0.3	0.3
Amortization of net actuarial loss (gain)	1.6	3.6	3.2	4.7
Curtailment and settlement loss (gain)			(0.1)	2.9
Less: expected participant contributions	0.0	(0.1)	0.0	(0.3)
Other pension and postretirement benefits (costs), net	(9.9)	(9.4)	(19.9)	(22.7)
Net periodic pension and OPEB cost	(6.2)	$ (4.7)	(12.5)	$ (13.6)
Maximum [Member]
Net periodic pension and OPEB costs:
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year	$ 10.0		$ 10.0