Fair Value Measurements - Change in Fair Values of Level 3 Financial Instruments (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Issuance of financial instrument	$ 0		$ 0
Securities Available for Sale [Member] | Other Securities [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	17,000,000	$ 17,000,000	10,000,000
Gains (Losses) Included in Earnings		0	0
Purchases		0	7,000,000
Sales		0	0
Settlements		0	0
Transfers Other		0
Transfers into Level 3		0	0
Transfers out of Level 3		0	0
End of Period Balance		17,000,000	17,000,000
Unrealized Gains (Losses) Included in Earnings		0	0
Other Investments [Member] | Principal Investments Direct [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	50,000,000	50,000,000	102,000,000
Gains (Losses) Included in Earnings		16,000,000	23,000,000
Purchases		0	5,000,000
Sales		(39,000,000)	(61,000,000)
Settlements		0	0
Transfers Other		0
Transfers into Level 3		0	0
Transfers out of Level 3		0	(19,000,000)
End of Period Balance		27,000,000	50,000,000
Unrealized Gains (Losses) Included in Earnings		2,000,000	3,000,000
Other Investments [Member] | Principal Other Indirect Investments [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	0	0
Gains (Losses) Included in Earnings		0
Purchases		0
Sales		(20,000,000)
Settlements		0
Transfers Other		0
Transfers into Level 3		20,000,000
Transfers out of Level 3		0
End of Period Balance		0	0
Unrealized Gains (Losses) Included in Earnings		(1,000,000)
Other Investments [Member] | Equity and Mezzanine Investments Direct [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	0	0	0
Gains (Losses) Included in Earnings			2,000,000
Purchases			0
Sales			(2,000,000)
Settlements			0
Transfers into Level 3			0
Transfers out of Level 3			0
End of Period Balance			0
Unrealized Gains (Losses) Included in Earnings			2,000,000
Interest Rate [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	16,000,000	16,000,000	13,000,000
Gains (Losses) Included in Earnings		4,000,000	2,000,000
Purchases		1,000,000	1,000,000
Sales		0	(1,000,000)
Settlements		0	0
Transfers Other		0
Transfers into Level 3		9,000,000	10,000,000
Transfers out of Level 3		(23,000,000)	(9,000,000)
End of Period Balance		7,000,000	16,000,000
Unrealized Gains (Losses) Included in Earnings		0	0
Credit [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	1,000,000	1,000,000	2,000,000
Gains (Losses) Included in Earnings		(13,000,000)	(12,000,000)
Purchases		1,000,000	0
Sales		0	0
Settlements		12,000,000	11,000,000
Transfers Other		0
Transfers into Level 3		0	0
Transfers out of Level 3		0	0
End of Period Balance		1,000,000	1,000,000
Unrealized Gains (Losses) Included in Earnings		0	0
Other Credit Derivatives [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	$ 0	0
Gains (Losses) Included in Earnings		0
Purchases		5,000,000
Sales		0
Settlements		0
Transfers Other		(3,000,000)
Transfers into Level 3		0
Transfers out of Level 3		0
End of Period Balance		2,000,000	$ 0
Unrealized Gains (Losses) Included in Earnings		$ 0