Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	0.00%	1.001%
Average Days To Maturity	0 days	184 days
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	1.37%	0.97%
Average Days To Maturity	370 days	400 days
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	1.766%	1.745%
Average Days To Maturity	392 days	318 days