Derivatives and Financial Instruments - Recorded Amounts and Estimated Fair Values of Long-term Debt and Derivative Financial Instruments (Detail) - USD ($) $ in Millions	1 Months Ended
	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Total debt		$ 839.6	$ 1,029.1
Carrying (Reported) Amount, Fair Value Disclosure
Derivative [Line Items]
Variable rate debt		236.1	353.6
Fixed rate debt		611.4	685.4
Total debt		847.5	1,039.0
Estimate of Fair Value, Fair Value Disclosure
Derivative [Line Items]
Variable rate debt		236.1	353.6
Fixed rate debt		695.4	732.2
Total debt		$ 931.5	$ 1,085.8
Senior Notes 2.450% due 2019 | Senior Notes
Derivative [Line Items]
Tender Offer	$ 363.4