Fair Value and Carrying Value of Long-Term Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Carrying amount	$ 3,418.1	$ 3,380.6
Estimated Fair Value	3,810.5	3,527.8
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Carrying amount	299.9	299.8
Estimated Fair Value	312.9	320.6
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount	516.9	505.8
Estimated Fair Value	563.9	551.2
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount	495.3	494.7
Estimated Fair Value	557.8	530.0
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount	495.1	494.6
Estimated Fair Value	568.8	533.8
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Carrying amount	455.6	449.4
Estimated Fair Value	463.4	454.3
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Carrying amount	597.3	597.2
Estimated Fair Value	743.7	617.7
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount	558.0	539.1
Estimated Fair Value	$ 600.0	$ 520.2