Netting Arrangements (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets:
Derivative assets, gross amount	$ 2,960	$ 4,422
Derivative assets, gross amount offset	(2,608)	(4,272)
Derivative assets, net amount presented in our condensed consolidated balance sheets	352	150
Derivative assets, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	(196)	(117)
Derivative assets, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	0	(1)
Derivative assets, net amount	156	32
Securities purchased under agreements to resell or similar arrangements, gross amount	27,350	44,670
Securities purchased under agreements to resell or similar arrangements, gross amount offset	0	0
Securities Purchased under Agreements to Resell, Not Subject to Master Netting Arrangement	27,350	44,670
Securities purchased under agreements to resell or similar arrangements, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	0	0
Securities purchased under agreements to resell or similar arrangements, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	(27,350)	(44,670)
Securities purchased under agreements to resell or similar arrangements, net amount	0	0
Total assets, gross amount	30,310	49,092
Total assets, gross amount offset	(2,608)	(4,272)
Total assets, net amount presented in our condensed consolidated balance sheets	27,702	44,820
Total assets, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	(196)	(117)
Total assets, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	(27,350)	(44,671)
Total assets, net amount	156	32
Liabilities:
Derivative liabilities, gross amount	(3,359)	(5,306)
Derivative liabilities, gross amount offset	2,756	4,979
Derivative liabilities, net amount presented in our condensed consolidated balance sheets	(603)	(327)
Derivative liabilities, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	196	117
Derivative liabilities, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	326	104
Derivative liabilities, net amount	(81)	(106)
Total liabilities, gross amount	(3,359)	(5,306)
Total liabilities, gross amount offset	2,756	4,979
Total liabilities, net amount presented in our condensed consolidated balance sheets	(603)	(327)
Total liabilities, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	196	117
Total liabilities, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	326	104
Total liabilities, net amount	(81)	(106)
Risk Management Derivatives
Assets:
Derivative assets, gross amount offset	(2,608)	(4,272)
Liabilities:
Derivative liabilities, gross amount offset	2,756	4,979
Mortgage commitment derivatives
Assets:
Derivative assets, gross amount	288	132
Derivative assets, gross amount offset	0	0
Derivative assets, net amount presented in our condensed consolidated balance sheets	288	132
Derivative assets, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	(196)	(117)
Derivative assets, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	0	(1)
Derivative assets, net amount	92	14
Liabilities:
Derivative liabilities, gross amount	(554)	(228)
Derivative liabilities, gross amount offset	0	0
Derivative liabilities, net amount presented in our condensed consolidated balance sheets	(554)	(228)
Derivative liabilities, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	196	117
Derivative liabilities, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	325	93
Derivative liabilities, net amount	(33)	(18)
Over the Counter [Member] | Risk Management Derivatives
Assets:
Derivative assets, gross amount	2,672	2,479
Derivative assets, gross amount offset	(2,630)	(2,464)
Derivative assets, net amount presented in our condensed consolidated balance sheets	42	15
Derivative assets, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	0	0
Derivative assets, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	0	0
Derivative assets, net amount	42	15
Liabilities:
Derivative liabilities, gross amount	(2,805)	(3,045)
Derivative liabilities, gross amount offset	2,757	2,957
Derivative liabilities, net amount presented in our condensed consolidated balance sheets	(48)	(88)
Derivative liabilities, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	0	0
Derivative liabilities, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	0	0
Derivative liabilities, net amount	(48)	(88)
Exchange Cleared [Member] | Risk Management Derivatives
Assets:
Derivative assets, gross amount	0	1,811
Derivative assets, gross amount offset	22	(1,808)
Derivative assets, net amount presented in our condensed consolidated balance sheets	22	3
Derivative assets, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	0	0
Derivative assets, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	0	0
Derivative assets, net amount	22	3
Liabilities:
Derivative liabilities, gross amount	0	(2,033)
Derivative liabilities, gross amount offset	(1)	2,022
Derivative liabilities, net amount presented in our condensed consolidated balance sheets	(1)	(11)
Derivative liabilities, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, financial instruments	0	0
Derivative liabilities, under master netting arrangements, amounts not offset in our condensed consolidated balance sheets, collateral	1	11
Derivative liabilities, net amount	$ 0	$ 0