Supplemental Cash Flow Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Open reverse repurchase commitments	$ 0	$ 0		$ 0
Dividends payable	58,400,000		$ 58,400,000	1,467,900,000	$ 655,100,000
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	21,000,000.0
Operating Lease, Payments	37,400,000
Income Taxes Paid	592,800,000	358,900,000
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	90,900,000	70,900,000
Cash and Cash Equivalents | ARX Holding Corp.
Open reverse repurchase commitments	$ 138,200,000	$ 155,900,000		$ 117,300,000