Supplemental Cash Flow Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Open reverse repurchase commitments	$ 0	$ 0		$ 0
Dividends payable on common shares	58,500,000	58,500,000		2,694,500,000
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	55,900,000	23,900,000
Operating Lease, Payments	44,800,000	44,700,000
Restricted Cash and Cash Equivalents	15,100,000	1,100,000		0
Income Taxes Paid	574,100,000	18,000,000.0
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	112,000,000.0	94,100,000
Collateral Against Unpaid Premiums
Restricted Cash and Cash Equivalents	14,800,000
Cash and Cash Equivalents | ARX Holding Corp.
Open reverse repurchase commitments	90,500,000	96,400,000		93,500,000
Common equities | Quarterly dividend [Member]
Dividends payable on common shares	$ 58,500,000	$ 58,500,000	$ 58,500,000	$ 58,600,000	$ 58,500,000	$ 58,500,000