Pension and Other Postretirement Benefits - Schedule of Expected Benefit Payments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
2013	$ 33.2
2014	34.9
2015	36.7
2016	38.5
2017	40.6
2018-2022	234.4
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
2013	2.6
2014	2.8
2015	3.0
2016	3.2
2017	3.3
2018-2022	17.4
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
2013	2.5
2014	2.5
2015	2.5
2016	2.5
2017	2.5
2018-2022	$ 12.0

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Amount of benefits expected to be paid in the five fiscal years after the fifth fiscal year following the latest fiscal year from a 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 (f)

-URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920

+ Details

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- Definition

Amount of benefits expected to be paid in the next fiscal year following the latest fiscal year from a defined benefit plan.

+ References

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Amount of benefits expected to be paid in the fifth fiscal year following the latest fiscal year from a defined benefit plan.

+ References

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Amount of benefits expected to be paid in the fourth fiscal year following the latest fiscal year from a defined benefit plan.

+ References

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Amount of benefits expected to be paid in the third fiscal year following the latest fiscal year from a defined benefit plan.

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Amount of benefits expected to be paid in the second fiscal year following the latest fiscal year from a defined benefit plan.

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