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Stockholders' Equity (Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax $ 361 $ (193) $ (145) $ (621) $ (205) $ 90 $ 121 $ 46 $ (598) $ 52 $ (370)
Amounts reclassified from accumulated OCI                 0 0 39
Agency Securities [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Beginning OCI Balance       $ (345)       $ (397) (345) (397) (66)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax                 (677) (31) (261)
Amounts reclassified from accumulated OCI                 79 83 (109)
Ending OCI Balance $ (943)       $ (345)       $ (943) $ (345) $ (397)