Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	98.50%	100.10%
Average Days To Maturity	83 days	184 days
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	105.60%	97.00%
Average Days To Maturity	264 days	400 days
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	172.00%	174.50%
Average Days To Maturity	246 days	318 days