Supplemental Cash Flow Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Open reverse repurchase commitments	$ 0	$ 0	$ 0
Dividends Payable	58,500,000	58,500,000	2,694,500,000
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	6,200,000	9,700,000
Operating Lease, Payments	24,500,000	22,700,000
Income Taxes Paid	800,000	0
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	74,300,000	56,400,000
Cash and Cash Equivalents | ARX Holding Corp.
Open reverse repurchase commitments	154,000,000.0	80,600,000	93,500,000
Common equities | Quarterly dividend [Member]
Dividends Payable	$ 58,500,000	$ 58,500,000	$ 58,600,000	$ 58,500,000