Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	0.00%	0.15%
Average Days To Maturity	0 days	86 days
Municipal bonds and notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	0.528%	0.528%
Average Days To Maturity	134 days	499 days
Corporate bonds and notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	1.828%	2.134%
Average Days To Maturity	348 days	390 days