Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 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	318 days	499 days
Corporate bonds and notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	1.705%	2.134%
Average Days To Maturity	366 days	390 days