Changes in Fair Value of Assets Measured Using Significant Unobservable Inputs (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 11	$ 37	$ 32	$ 37
Realized in earnings	0	0	(1)	0
Unrealized in other comprehensive income	0	1	0	1
Purchases	0	0	0	0
Sales	0	0	(20)	0
Settlements	0	0	0	0
Ending Balance	11	38	11	38
Private Placement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	6	24	24	24
Realized in earnings	0	0	(1)	0
Unrealized in other comprehensive income	0	1	0	1
Purchases	0	0	0	0
Sales	0	0	(17)	0
Settlements	0	0	0	0
Ending Balance	6	25	6	25
Auction Rate Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	5	13	8	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	(3)	0
Settlements	0	0	0	0
Ending Balance	$ 5	$ 13	$ 5	$ 13