Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	123.60%	100.10%
Average Days To Maturity	228 days	184 days
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	121.30%	97.00%
Average Days To Maturity	325 days	400 days
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	171.50%	174.50%
Average Days To Maturity	295 days	318 days