Fair Value Measurements - Changes in balances of Level 3 financial liabilities carried at fair value (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial liabilities, Beginning Balance		$ (2,542)	$ (1,759)
Total realized/unrealized gains (losses) included in Net income		63	(147)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		0	0
Purchases and issuances		(103)	(79)
Sales and settlements		33	22
Transfer into Level 3		0	0
Transfer out of Level 3		0	0
Financial liabilities, Ending Balance	[1]	(2,549)	(1,963)
Embedded derivatives
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial liabilities, Beginning Balance		(2,542)	(1,759)
Total realized/unrealized gains (losses) included in Net income		63	(147)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		0	0
Purchases and issuances		(103)	(79)
Sales and settlements		33	22
Transfer into Level 3		0	0
Transfer out of Level 3		0	0
Financial liabilities, Ending Balance		$ (2,549)	$ (1,963)

[1] As previously discussed, these tables exclude the portion of MIE liabilities allocated to Level 3, which are derived from the fair value of the MIE assets.