Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
US Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Average Coupon Rate	0.00%	1.37%
Average Days To Maturity	0 days	370 days
Corporate Bonds and Notes [Member]
Debt Securities, Available-for-sale [Line Items]
Average Coupon Rate	0.00%	1.766%
Average Days To Maturity	0 days	392 days