Summary of Total Indebtedness (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Long-term debt	$ 3,401.0	$ 2,547.3
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Long-term debt	300.0	300.0
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt	507.8	503.8
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt	497.2	496.9
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt	497.7	497.5
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Long-term debt	451.8	450.7
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Long-term debt	603.4	$ 298.4
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt	$ 543.1