Pledged Assets (Assets Pledged from Counterparties) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Offsetting Assets and Liabilities [Line Items]
Obligation to Return Securities Borrowed Under Reverse Repurchase Agreements at Fair Value	$ 4,886	$ 9,543
Restricted Cash and Cash Equivalents	1,978	451
Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted Cash and Cash Equivalents	27	116
Restricted Cash and Securities Pledged	4,880	10,216
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	(60)	(1)
Restricted Cash and Cash Equivalents	0
Securities Sold under Agreements to Repurchase [Member] | Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	(60)	(1)
Derivative [Member] | Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted Cash and Securities Pledged	27	116
US Treasury Securities [Member] | Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	(4,853)	(10,100)
Reverse Repurchase Agreements [Member] | Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	(4,913)	(10,099)
Reverse Repurchase Agreements [Member] | US Treasury Securities [Member] | Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	(4,913)	(10,099)
Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted Cash and Cash Equivalents	938	340
Derivative [Member] | US Treasury Securities [Member] | Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	0	0
Derivative [Member] | Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted Cash and Cash Equivalents	$ 27	$ 116