Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net periodic pension and OPEB costs:
Service cost - benefits earned during the year	$ 4.5	$ 2.3	$ 13.6	$ 7.3
Interest cost on projected benefit obligation	59.5	32.5	177.0	99.9
Expected return on plan assets	(68.1)	(38.0)	(200.9)	(118.0)
Amortization of prior service cost (benefit)	0.2	0.3	0.5	0.6
Amortization of net actuarial loss (gain)	8.1	7.4	20.9	23.0
OPEB Recognized Net (Gain) Loss Due to Curtailments			(2.9)	0.0
Defined Benefit Plan Expected Participant Contributions	(0.2)	(0.1)	(0.5)	(0.4)
Net periodic pension and OPEB cost	4.0	4.4	7.7	12.4
Contributions paid to defined benefit plans			308.0
Defined Benefit Plans
Net periodic pension and OPEB costs:
Service cost - benefits earned during the year	1.9	1.9	5.6	5.7
Interest cost on projected benefit obligation	51.8	31.0	154.1	95.5
Expected return on plan assets	(68.2)	(38.0)	(201.1)	(118.0)
Amortization of prior service cost (benefit)	0.2	0.2	0.5	0.6
Amortization of net actuarial loss (gain)	8.1	7.4	20.9	23.0
OPEB Recognized Net (Gain) Loss Due to Curtailments			0.0	0.0
Defined Benefit Plan Expected Participant Contributions	(0.2)	(0.1)	(0.5)	(0.4)
Net periodic pension and OPEB cost	(6.4)	2.4	(20.5)	6.4
Contributions paid to defined benefit plans	200.0
Other Postretirement Benefits
Net periodic pension and OPEB costs:
Service cost - benefits earned during the year	2.6	0.4	8.0	1.6
Interest cost on projected benefit obligation	7.7	1.5	22.9	4.4
Expected return on plan assets	0.1	0.0	0.2	0.0
Amortization of prior service cost (benefit)	0.0	0.1	0.0	0.0
Amortization of net actuarial loss (gain)	0.0	0.0	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	0.0	0.0
Net periodic pension and OPEB cost	$ 10.4	$ 2.0	28.2	$ 6.0
Maximum [Member]
Net periodic pension and OPEB costs:
Defined Benefit Plan, Expected Contributions in Current Fiscal Year			$ 310.0

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OPEB Recognized Net (Gain) Loss Due to Curtailments

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The amount of gains or losses recognized in net periodic benefit cost.

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The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation.

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Amount of contributions made by the employer to defined benefit plans.

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Amount of the employer's best estimate of contributions expected to be paid to the plan in the current fiscal period.

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An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

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The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

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The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

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The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

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