Accumulated Other Comprehensive Income - Amounts Reclassified Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Postretirement Plans:
Total reclassification for the period	$ 34.9	$ 38.7
Postretirement Plans
Postretirement Plans:
Amortization of postretirement plan items, before tax	35.9	40.0
Income tax benefit	(1.0)	(1.3)
Total reclassification for the period	34.9
Amortization of prior service cost
Postretirement Plans:
Amortization of postretirement plan items, before tax	(1.5)	(1.8)
Amortization of actuarial losses
Postretirement Plans:
Amortization of postretirement plan items, before tax	$ 37.4	$ 41.8

Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e640-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 17A
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL34724391-108580

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 20
-Section 45
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109238882&loc=d3e39076-109324

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration