Pledged Assets (Assets Pledged from Counterparties) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Offsetting Assets and Liabilities [Line Items]
Document Period End Date	Dec. 31, 2018
Obligation to Return Securities Borrowed Under Reverse Repurchase Agreements at Fair Value	$ 21,431	$ 10,467
Restricted Cash and Cash Equivalents	599	317
Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted Cash and Cash Equivalents	129	82
Restricted Cash and Securities Pledged	22,077	10,935
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	37	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	37	0
Derivative [Member] | Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted Cash and Securities Pledged	164	82
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	21,948	10,853
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	21,876	10,853
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	21,876	10,853
Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted Cash and Cash Equivalents	522	281
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	35	0
Derivative [Member] | Assets Pledged to Us [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted Cash and Cash Equivalents	$ 129	$ 82