Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Net periodic pension and OPEB costs:
Service cost - benefits earned during the year	$ 4.4	$ 2.4
Interest cost on projected benefit obligation	59.1	33.2
Expected return on plan assets	(66.0)	(39.5)
Amortization of prior service cost (benefit)	0.2	0.2
Amortization of net actuarial loss (gain)	5.0	7.7
OPEB Recognized Net (Gain) Loss Due to Curtailments	2.9	0.0
Defined Benefit Plan Expected Participant Contributions	(0.2)	(0.1)
Net periodic pension and OPEB cost	(0.4)	3.9
Contributions paid to defined benefit plans	36.0
Defined Benefit Plans
Net periodic pension and OPEB costs:
Service cost - benefits earned during the year	1.8	1.8
Interest cost on projected benefit obligation	51.5	31.9
Expected return on plan assets	(66.1)	(39.5)
Amortization of prior service cost (benefit)	0.2	0.2
Amortization of net actuarial loss (gain)	5.0	7.7
OPEB Recognized Net (Gain) Loss Due to Curtailments	0.0	0.0
Defined Benefit Plan Expected Participant Contributions	(0.2)	(0.1)
Net periodic pension and OPEB cost	(7.8)	2.0
Other Postretirement Benefits
Net periodic pension and OPEB costs:
Service cost - benefits earned during the year	2.6	0.6
Interest cost on projected benefit obligation	7.6	1.3
Expected return on plan assets	0.1	0.0
Amortization of prior service cost (benefit)	0.0	0.0
Amortization of net actuarial loss (gain)	0.0	0.0
OPEB Recognized Net (Gain) Loss Due to Curtailments	2.9	0.0
Defined Benefit Plan Expected Participant Contributions	0.0	0.0
Net periodic pension and OPEB cost	7.4	$ 1.9
Minimum [Member]
Net periodic pension and OPEB costs:
Defined Benefit Plan, Expected Contributions in Current Fiscal Year	100.0
Maximum [Member]
Net periodic pension and OPEB costs:
Defined Benefit Plan, Expected Contributions in Current Fiscal Year	$ 120.0