Accumulated Other Comprehensive Income - Amounts Reclassified Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Postretirement Plans:
Total reclassification for the period	$ 34.8	$ 38.6	$ 69.7	$ 77.3
Postretirement Plans
Postretirement Plans:
Amortization of postretirement plan items, before tax	35.8	39.8	71.7	79.8
Income tax benefit	(1.0)	(1.2)	(2.0)	(2.5)
Total reclassification for the period			69.7
Amortization of prior service cost
Postretirement Plans:
Amortization of postretirement plan items, before tax	(1.5)	(1.8)	(3.0)	(3.6)
Amortization of actuarial losses
Postretirement Plans:
Amortization of postretirement plan items, before tax	$ 37.3	$ 41.6	$ 74.7	$ 83.4

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=118930883&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=118930883&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

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