Fair Value Measurements - Change in Fair Values of Level 3 Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Securities Available for Sale [Member] | Other Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 10		$ 10
Purchases	7	$ 10	7	$ 10
Ending Balance	17	10	17	10
Other Investments [Member] | Principal Investments Direct [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	70	146	102	141
Gains (Losses) Included in Earnings	4	(2)	20	9
Purchases	3		5	1
Sales	(11)	(29)	(61)	(36)
Ending Balance	66	115	66	115
Unrealized Gains (Losses) Included in Earnings	3	(2)		18
Other Investments [Member] | Principal Investments Indirect [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	282	399	302	413
Gains (Losses) Included in Earnings	8	9	33	(49)
Purchases	2	3	6	8
Sales	(21)	(58)	(70)	(117)
Ending Balance	271	353	271	353
Unrealized Gains (Losses) Included in Earnings	(8)	(4)	(23)	8
Other Investments [Member] | Equity and Mezzanine Investments Direct [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Gains (Losses) Included in Earnings			2
Sales			(2)
Unrealized Gains (Losses) Included in Earnings			2
Other Investments [Member] | Equity and Mezzanine Investments Indirect [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	9	16	10	23
Gains (Losses) Included in Earnings			6	(1)
Sales		(3)	(7)	(9)
Ending Balance	9	13	9	13
Unrealized Gains (Losses) Included in Earnings			6	(1)
Other Investments [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	4	4	4	4
Ending Balance	4	4	4	4
Interest Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	16	20	13	25
Gains (Losses) Included in Earnings	6		5	2
Purchases			1	3
Sales				(2)
Transfers into Level 3	2	2	10	7
Transfers out of Level 3	(2)	(3)	(7)	(16)
Ending Balance	22	19	22	19
Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance		1
Purchases		(1)		(1)
Transfers into Level 3				1
Transfers out of Level 3		(1)		(1)
Ending Balance		(1)		(1)
Credit [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	3	3	2	3
Gains (Losses) Included in Earnings	(3)	(2)	(7)	(7)
Sales	8		8
Settlements	(5)	2		7
Ending Balance	$ 3	$ 3	$ 3	$ 3