Derivative Financial Instruments - Master Netting (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Amounts subject to master netting arrangements	$ 1,708
Derivative Asset, Fair Value, Amount Offset Against Collateral [Abstract]
Derivative Asset, Collateral, Obligation to Return Cash, Offset	499
Derivative, Collateral, Obligation to Return Securities	18
Derivative Liability, net	366	$ 118
Fair Value, Gain	4,260	246
Derivative Liability, Gross	3,336	247
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	2,970
Derivative, Collateral, Right to Reclaim Securities	33
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	333
Unsecured positions in a net gain with counterparties after collateral postings	2,035
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(2,207)
Net amount	2,053	$ 146
Exchange Traded
Derivative Asset, Fair Value, Amount Offset Against Collateral [Abstract]
Derivative, Collateral, Obligation to Return Securities	0
Derivative Liability, net	0
Fair Value, Gain	606
Derivative Liability, Gross	204
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	204
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0
Unsecured positions in a net gain with counterparties after collateral postings	402
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(204)
Net amount	(402)
Derivatives Not Subject to Master Netting Arrangement
Derivative Asset, Fair Value, Amount Offset Against Collateral [Abstract]
Derivative, Collateral, Obligation to Return Securities	1
Derivative Liability, net	188
Fair Value, Gain	138
Derivative Liability, Gross	193
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	5
Derivative, Collateral, Right to Reclaim Securities	11
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	177
Unsecured positions in a net gain with counterparties after collateral postings	137
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	0
Net amount	(138)
Derivatives Subject to Master Netting Arrangements
Derivative Asset, Fair Value, Amount Offset Against Collateral [Abstract]
Derivative, Collateral, Obligation to Return Securities	17
Derivative Liability, net	178
Fair Value, Gain	3,516
Derivative Liability, Gross	2,939
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	2,761
Derivative, Collateral, Right to Reclaim Securities	22
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	156
Unsecured positions in a net gain with counterparties after collateral postings	1,496
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(2,003)
Net amount	$ (1,513)