Summary of Total Indebtedness (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Fair value of interest rate swap	$ 44.9	$ 41.8
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	6.06%	6.06%
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Fair value of interest rate swap	$ 17.1	$ 5.8
Unamortized discount	$ 1.7	$ 2.0
Notes Payable, interest rate	5.50%	5.50%
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized discount	$ 2.9	$ 3.1
Notes Payable, interest rate	4.50%	4.50%
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized discount	$ 2.4	$ 2.5
Notes Payable, interest rate	4.875%	4.875%
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Fair value of interest rate swap	$ 8.4	$ 1.4
Unamortized discount	$ 0.5	$ 0.7
Notes Payable, interest rate	2.75%	2.75%
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Unamortized discount	$ 1.6	$ 1.6
Notes Payable, interest rate	5.25%	5.25%
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	1.75%	1.75%