Derivatives and Financial Instruments - Recorded Amounts and Estimated Fair Values of Long-term Debt and Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Total debt, excluding unamortized issuance costs and discounts	$ 1,511.2
Recorded Amount
Derivative [Line Items]
Variable rate debt	133.2	$ 976.3
Fixed rate debt	1,378.0	3,320.7
Total debt, excluding unamortized issuance costs and discounts	1,511.2	4,297.0
Fair Value
Derivative [Line Items]
Variable rate debt	133.2	976.3
Fixed rate debt	1,424.3	3,427.1
Total debt, excluding unamortized issuance costs and discounts	$ 1,557.5	$ 4,403.4