Fair Value and Carrying Value of Long-Term Debt (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long term debt	$ 5,417.7	$ 5,410.6
Estimated Fair Value	5,599.6	5,702.1
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Long term debt	492.5	497.8
Estimated Fair Value	527.8	537.9
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Long term debt	494.1	495.5
Estimated Fair Value	521.9	535.6
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Long term debt	495.9	495.8
Estimated Fair Value	531.1	547.8
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Long term debt	444.6	446.5
Estimated Fair Value	448.9	452.8
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Long term debt	597.7	597.6
Estimated Fair Value	692.2	722.4
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Long term debt	611.4	596.8
Estimated Fair Value	630.5	617.7
2017 Senior Notes [Member]
Debt Instrument [Line Items]
Long term debt	495.8	495.5
Estimated Fair Value	490.4	500.0
2017 Floating Rate Senior Notes [Member]
Debt Instrument [Line Items]
Long term debt	299.7	299.5
Estimated Fair Value	300.2	300.2
2.625% 2017 Notes, due 2023 [Member]
Debt Instrument [Line Items]
Long term debt	495.6	495.4
Estimated Fair Value	481.9	494.8
3.25% 2017 Notes, due 2028 [Member]
Debt Instrument [Line Items]
Long term debt	491.0	490.9
Estimated Fair Value	475.3	493.6
2017 Term Loan Facility, due 2020 [Member]
Debt Instrument [Line Items]
Long term debt	499.4	499.3
Estimated Fair Value	$ 499.4	$ 499.3