Summary of Total Indebtedness (Parenthetical) (Detail)	Jun. 30, 2018	Dec. 31, 2017
5.50% 2010 Senior Notes, due 2020 [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	5.50%	5.50%
4.50% 2012 Senior Notes, due 2022 [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	4.50%	4.50%
4.875% 2013 Senior Notes, due 2024 [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	4.875%	4.875%
2.75% 2014 Senior Notes (5-Year), due 2019 [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	2.75%	2.75%
5.25% 2014 Senior Notes (30-Year), due 2044 [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	5.25%	5.25%
1.75% 2015 Senior Notes, due 2027 [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	1.75%	1.75%
2.75% 2017 Senior Notes, due 2021 [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%
3.25% 2018 Notes, due 2021 [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	3.25%