Long-Term Compensation Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
RSUI
Executive Compensation Plan Expense [Line Items]
Compensation expense recognized	$ 29.0	$ 37.4	$ 11.7
Income tax (expenses) benefit related to share based compensation expense	6.1	7.8	2.4
Parent Company
Executive Compensation Plan Expense [Line Items]
Compensation expense recognized	4.3	38.7	(12.0)
Income tax (expenses) benefit related to share based compensation expense	0.9	8.1	(2.5)
Fair value of common stocks issued to satisfy share based compensations	5.0	2.4	4.3
Compensation costs paid in cash	12.8	2.2	17.1
Reinsurance Segment
Executive Compensation Plan Expense [Line Items]
Compensation expense recognized	44.3	48.7	35.6
Income tax (expenses) benefit related to share based compensation expense	$ 9.3	$ 10.2	$ 7.5

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

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