Investments - Average Coupon Rate Percentage and Average Days to Maturity of Debt (Detail)	Sep. 30, 2013	Dec. 31, 2012
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	0.139%	0.305%
Average Term to Maturity	62 days	138 days
Corporate bonds and notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Average Coupon Rate	2.311%	2.965%
Average Term to Maturity	418 days	112 days