Derivatives and Financial Instruments - Recorded Amounts and Estimated Fair Values of Long-term Debt and Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Total debt, excluding unamortized issuance costs and discounts	$ 4,859.9
Recorded Amount
Derivative [Line Items]
Variable rate debt	1,500.1	$ 1,360.9
Fixed rate debt	3,359.8	3,349.1
Total debt, excluding unamortized issuance costs and discounts	4,859.9	4,710.0
Fair Value
Derivative [Line Items]
Variable rate debt	1,500.1	1,360.9
Fixed rate debt	3,399.0	3,395.4
Total debt, excluding unamortized issuance costs and discounts	$ 4,899.1	$ 4,756.3