Summary of Total Indebtedness (Parenthetical) (Detail)	Mar. 31, 2018	Dec. 31, 2017
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%
2017 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	2.75%	2.75%
2.625% 2017 Notes, due 2023 [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	2.65%	2.65%
3.25% 2017 Notes, due 2028 [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	3.25%	3.25%