Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	0.823%
Average Days To Maturity	631 days
Municipal Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate		0.528%
Average Days To Maturity		134 days
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	1.802%	1.828%
Average Days To Maturity	353 days	348 days