Derivatives and Financial Instruments - Recorded Amounts and Estimated Fair Values of Long-term Debt and Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Total debt, excluding unamortized issuance costs and discounts	$ 2,686.2
Recorded Amount
Derivative [Line Items]
Variable rate debt	480.1	$ 62.4
Fixed rate debt	2,206.1	1,385.1
Total debt, excluding unamortized issuance costs and discounts	2,686.2	1,447.5
Fair Value
Derivative [Line Items]
Variable rate debt	480.1	62.4
Fixed rate debt	2,250.8	1,424.0
Total debt, excluding unamortized issuance costs and discounts	$ 2,730.9	$ 1,486.4