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Investment Securities (Summary of Investment in Agency Security) (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Amortized cost $ 46,744 $ 46,866
Mortgage Backed Securities Amortized Cost 46,744 46,866
Fair value 46,566 46,499
Total agency MBS, amortized cost 46,164 46,580
Total agency MBS, at fair value 45,963 46,211
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure 46,566 46,499
Mortgage-backed Securities, Issued by Private Enterprises [Member]    
Amortized cost 8 102
Fair value 8 101
Commercial Mortgage Backed Securities [Member]    
Amortized cost 23 23
Fair value 23 23
Credit Risk Transfer Securities [Member]    
Interest-only and principal-only strips, amortized cost 549 161
Interest-only and principal-only strips, fair value 572 164
Fixed Income Securities [Member] | Agency Securities [Member]    
Amortized cost 44,883 45,145
Fair value 44,643 44,736
Adjustable-Rate [Member] | Agency Securities [Member]    
Amortized cost 327 371
Fair value 334 379
Collateralized Mortgage Obligations [Member] | Agency Securities [Member]    
Amortized cost 714 796
Fair value 722 801
Interest Only And Principal Only Strip [Member] | Agency Securities [Member]    
Interest-only and principal-only strips, amortized cost 240 268
Interest-only and principal-only strips, fair value $ 264 $ 295