Notes Payable - Components of Notes Payable (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2016	Nov. 05, 2015	Mar. 17, 2014	Apr. 21, 2011
Series A senior notes [Member] | 4.03% Series A senior notes due in 2018 [Member]
Debt Instrument [Line Items]
Senior Notes, interest rate	4.03%			4.03%
Senior Notes, year of maturity	2018
Series B Senior Notes | 3.68% Series B senior notes due in 2021 [Member]
Debt Instrument [Line Items]
Senior Notes, interest rate	3.68%		3.68%
Senior Notes, year of maturity	2021
Series C Senior Notes | 3.84% Series C senior notes due in 2022 [Member]
Debt Instrument [Line Items]
Senior Notes, interest rate	3.84%	3.84%
Senior Notes, year of maturity	2022