Derivatives and Financial Instruments - Recorded Amounts and Estimated Fair Values of Long-term Debt and Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Total debt, excluding unamortized issuance costs and discounts	$ 1,708.0
Recorded Amount
Derivative [Line Items]
Variable rate debt	0.3	$ 976.3
Fixed rate debt	1,707.7	3,320.7
Total debt, excluding unamortized issuance costs and discounts	1,708.0	4,297.0
Fair Value
Derivative [Line Items]
Variable rate debt	0.3	976.3
Fixed rate debt	1,735.0	3,427.1
Total debt, excluding unamortized issuance costs and discounts	$ 1,735.3	$ 4,403.4