Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	1.001%	0.553%
Average Days To Maturity	184 days	154 days
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	0.97%	0.865%
Average Days To Maturity	400 days	599 days
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	1.745%	1.757%
Average Days To Maturity	318 days	331 days