Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	0.978%	0.553%
Average Days To Maturity	253 days	154 days
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	0.979%	0.865%
Average Days To Maturity	512 days	599 days
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	1.643%	1.757%
Average Days To Maturity	330 days	331 days