Employee Benefits - Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Costs
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	$ 44.6	$ 45.4	$ 30.4
Interest cost	121.8	130.8	94.3
Expected return on plan assets	(195.7)	(195.9)	(155.6)
Plan settlement	9.0	16.5	0.0
Plan curtailment	0.0	0.0	(0.3)
Amortization of prior service cost (credit)	2.9	3.4	2.2
Amortization of net actuarial loss	86.1	82.9	68.5
Net periodic benefit cost (credit)	68.7	83.1	39.5
OPEB Costs
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	24.1	26.1	20.7
Interest cost	32.9	37.0	26.7
Expected return on plan assets	(55.5)	(52.7)	(39.6)
Plan settlement	0.0	0.0	0.0
Plan curtailment	0.0	0.0	0.0
Amortization of prior service cost (credit)	(12.3)	(9.4)	(6.4)
Amortization of net actuarial loss	3.1	8.5	3.9
Net periodic benefit cost (credit)	$ (7.7)	$ 9.5	$ 5.3

Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan.

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Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan.

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Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan.

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Amount of cost recognized for passage of time related to defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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-Publisher FASB
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-Section 50
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Amount of net periodic benefit cost (credit) for defined benefit plan.

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Amount of gain (loss) recognized in net periodic benefit (cost) credit from event reducing expected years of future service of present employees or eliminating accrual of defined benefits for some or all future services of present employees.

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Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
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-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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