Fair Value and Carrying Value of Long-Term Debt (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying Amount		$ 3,363.0
Estimated Fair Value		3,584.9
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Carrying Amount		300.0
Estimated Fair Value		308.9
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	$ 497.8	502.6
Estimated Fair Value	537.9	548.3
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	495.5	495.3
Estimated Fair Value	535.6	535.3
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	495.8	495.2
Estimated Fair Value	547.8	539.9
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Carrying Amount	446.5	448.8
Estimated Fair Value	452.8	456.2
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Carrying Amount	597.6	597.4
Estimated Fair Value	722.4	661.5
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	596.8	523.7
Estimated Fair Value	617.7	$ 534.8
2017 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	495.5
Estimated Fair Value	500.0
2017 Floating Rate Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	299.5
Estimated Fair Value	300.2
2.625% 2017 Private Placement Notes, due 2023 [Member]
Debt Instrument [Line Items]
Carrying Amount	495.4
Estimated Fair Value	494.8
3.25% 2017 Private Placement Notes, due 2028 [Member]
Debt Instrument [Line Items]
Carrying Amount	490.9
Estimated Fair Value	493.6
2017 Term Loan Facility, due 2020 [Member]
Debt Instrument [Line Items]
Carrying Amount	499.3
Estimated Fair Value	$ 499.3