Derivatives and Financial Instruments - Recorded Amounts and Estimated Fair Values of Long-term Debt and Derivative Financial Instruments (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Total debt		$ 1,029.1	$ 787.6
Carrying (Reported) Amount, Fair Value Disclosure
Derivative [Line Items]
Variable rate debt		353.6	102.2
Fixed rate debt		685.4	690.5
Total debt		1,039.0	792.7
Estimate of Fair Value, Fair Value Disclosure
Derivative [Line Items]
Variable rate debt		353.6	102.2
Fixed rate debt		732.2	691.8
Total debt		$ 1,085.8	$ 794.0
Senior Notes 2.450% due 2019 | Senior Notes
Derivative [Line Items]
Tender Offer	$ 363.4