Fair Value and Carrying Value of Long-Term Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Estimated Fair Value	$ 3,244.6	$ 2,772.7
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Notes Payable	300.0	300.0
Estimated Fair Value	324.9	334.6
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	515.4	503.8
Estimated Fair Value	559.0	564.4
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	497.1	496.9
Estimated Fair Value	530.2	537.1
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	497.6	497.5
Estimated Fair Value	531.2	548.4
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Notes Payable	457.9	450.7
Estimated Fair Value	456.7	454.3
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Notes Payable	298.4	298.4
Estimated Fair Value	311.3	$ 333.9
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	558.1
Estimated Fair Value	$ 531.3