Fair Value of Financial Investments and Liabilities - Additional Information (Detail) (USD $)			3 Months Ended	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012 Assets at Fair Value (Level 3) [Member]	Sep. 30, 2012 Assets at Fair Value (Level 3) [Member]
Investment Holdings [Line Items]
Aggregate carrying value of remaining private equity funds			$ 4,000,000	$ 4,000,000
Impairment charges - private equity funds			2,000,000	2,000,000
Estimated market value of long-term and short-term debt	3,700,000,000	4,000,000,000
Aggregate carrying value of long-term and short-term debt	$ 3,400,000,000	$ 3,600,000,000