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, 2018	Dec. 31, 2017
Derivative [Line Items]
Total debt		$ 787.6	$ 1,440.7
Carrying (Reported) Amount, Fair Value Disclosure
Derivative [Line Items]
Variable rate debt		102.2	62.4
Fixed rate debt		690.5	1,385.1
Total debt		792.7	1,447.5
Estimate of Fair Value, Fair Value Disclosure
Derivative [Line Items]
Variable rate debt		102.2	62.4
Fixed rate debt		691.8	1,424.0
Total debt		$ 794.0	$ 1,486.4
Senior Notes 2.450% due 2019 | Senior Notes
Derivative [Line Items]
Tender Offer	$ 363.4