Long-Term Compensation Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock Units (RSUs) | RSUI
Executive Compensation Plan Expense [Line Items]
Compensation expense recognized	$ 26.3	$ 25.4	$ 22.7
Income tax benefit related to share based compensation expense	5.5	8.9	7.9
Parent Company
Executive Compensation Plan Expense [Line Items]
Compensation expense recognized	17.1	17.1	19.7
Income tax benefit related to share based compensation expense	3.6	6.0	6.9
Fair value of common stocks issued to satisfy share based compensations	7.1	5.7	4.7
Compensation costs paid in cash	19.3	14.4	10.9
Reinsurance Segment | Book Value Unit Plan Bvu And Midterm Incentive Plan Grant
Executive Compensation Plan Expense [Line Items]
Compensation expense recognized	46.6	50.7	54.9
Income tax benefit related to share based compensation expense	$ 9.8	$ 17.7	$ 19.2