Changes in Fair Value of Assets Measured Using Significant Unobservable Inputs (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 37	$ 36	$ 37	$ 38
Realized in earnings	0	0	0	0
Unrealized in other comprehensive income	1	1	1	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	(1)
Ending Balance	38	37	38	37
Private Placement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	24	23	24	25
Realized in earnings	0	0	0	0
Unrealized in other comprehensive income	1	1	1	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	(1)
Ending Balance	25	24	25	24
Auction Rate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	13	13	13	13
Realized in earnings	0	0	0	0
Unrealized in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Ending Balance	$ 13	$ 13	$ 13	$ 13