Pension and Postretirement Plans - Amounts Recognized in AOCI (Details) - USD ($) $ in Millions	4 Months Ended	8 Months Ended	12 Months Ended
Dec. 31, 2021	Sep. 07, 2021	Dec. 31, 2020
Pension Plans, Defined Benefit [Member]
Prior service cost recognized:
Reclassification adjustments	$ 0.0	$ 0.0	$ 0.0
Actuarial gain (loss) recognized:
Reclassification adjustments	0.0	15.6	21.9
Actuarial gain (loss) arising during the period	2.7	29.5	(7.1)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Prior service cost recognized:
Reclassification adjustments	0.0	(1.7)	(2.5)
Actuarial gain (loss) recognized:
Reclassification adjustments	0.0	0.0	1.2
Actuarial gain (loss) arising during the period	$ 0.7	$ 4.0	$ 3.6

Amount, before tax, of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e637-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
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Reference 4: 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=124509347&loc=SL34724391-108580

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