Long-Term Compensation Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Stock Units (RSUs) \| RSUI
Executive Compensation Plan Expense [Line Items]
Compensation expense recognized	$ 37.4	$ 11.7	$ 26.3
Income tax (expenses) benefit related to share based compensation expense	7.8	2.4	5.5
Parent Company
Executive Compensation Plan Expense [Line Items]
Compensation expense recognized	38.7	(12.0)	17.1
Income tax (expenses) benefit related to share based compensation expense	8.1	(2.5)	3.6
Fair value of common stocks issued to satisfy share based compensations	2.4	4.3	7.1
Compensation costs paid in cash	2.2	17.1	19.3
Reinsurance Segment \| Book Value Unit Plan Bvu And Midterm Incentive Plan Grant
Executive Compensation Plan Expense [Line Items]
Compensation expense recognized	48.7	35.6	46.6
Income tax (expenses) benefit related to share based compensation expense	$ 10.2	$ 7.5	$ 9.8

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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