Derivatives and Financial Instruments - Recorded Amounts and Estimated Fair Values (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Total debt	$ 1,375.2
Recorded Amount
Derivative [Line Items]
Variable rate debt	686.2	$ 102.2
Fixed rate debt	689.0	690.5
Total debt	1,375.2	792.7
Fair Value
Derivative [Line Items]
Variable rate debt	686.2	102.2
Fixed rate debt	694.6	691.8
Total debt	$ 1,380.8	$ 794.0