Fair Value and Carrying Value of Long-Term Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying amount	$ 5,406.6	$ 3,363.0
Estimated Fair Value	5,695.8	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	501.7	502.6
Estimated Fair Value	544.8	548.3
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount	496.1	495.3
Estimated Fair Value	538.7	535.3
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount	495.6	495.2
Estimated Fair Value	551.1	539.9
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Carrying amount	448.4	448.8
Estimated Fair Value	455.8	456.2
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Carrying amount	597.6	597.4
Estimated Fair Value	701.8	661.5
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount	587.6	523.7
Estimated Fair Value	607.9	$ 534.8
2017 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount	495.2
Estimated Fair Value	503.2
2017 Floating Rate Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount	299.3
Estimated Fair Value	300.5
2.625% 2017 Private Placement Notes, due 2023 [Member]
Debt Instrument [Line Items]
Carrying amount	495.2
Estimated Fair Value	496.8
3.25% 2017 Private Placement Notes, due 2028 [Member]
Debt Instrument [Line Items]
Carrying amount	490.7
Estimated Fair Value	496.0
2017 Term Loan Facility, due 2020 [Member]
Debt Instrument [Line Items]
Carrying amount	499.2
Estimated Fair Value	$ 499.2