Summary of Total Indebtedness (Parenthetical) (Detail)	Sep. 30, 2016	Dec. 31, 2015
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]
Notes Payable, interest rate	5.50%	5.50%
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	4.50%	4.50%
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	4.875%	4.875%
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	2.75%	2.75%
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	5.25%	5.25%
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	1.75%	1.75%