Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Capped eligible compensation	$ 1.0
Maximum allowed investment percentage in any single issuer in pension plan assets at time of purchase	10.00%
SERP
Retirement Benefits [Abstract]
Defined benefit plan contributions	$ 21.6	$ 52.8	$ 21.3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions	$ 21.6	$ 52.8	$ 21.3

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration