Investment Securities Securities by Credit Rating (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Securities Remeasured at Fair Value [Member] | Credit Risk Transfer Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Remeasured at Fair Value Through Earnings, Debt Securities	$ 572	$ 164
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Securities	37,030	46,017
Available-for-sale Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Securities	8	101
Available-for-sale Securities [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Available-for-sale Securities [Member] | Credit Risk Transfer Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Agency securities remeasured at fair value through earnings [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Remeasured at Fair Value Through Earnings, Debt Securities	9,536	482
Agency securities remeasured at fair value through earnings [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Remeasured at Fair Value Through Earnings, Debt Securities	0	0
Agency securities remeasured at fair value through earnings [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Remeasured at Fair Value Through Earnings, Debt Securities	23	23
BBB Rating [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Remeasured at Fair Value Through Earnings, Debt Securities	23	23
BB Rating [Member] | Credit Risk Transfer Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Remeasured at Fair Value Through Earnings, Debt Securities	78
AAA Rating [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Remeasured at Fair Value Through Earnings, Debt Securities	8	99
B Rating [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Remeasured at Fair Value Through Earnings, Debt Securities	0	2
B Rating [Member] | Credit Risk Transfer Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Remeasured at Fair Value Through Earnings, Debt Securities	490	$ 164
Not Rated [Member] | Credit Risk Transfer Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Remeasured at Fair Value Through Earnings, Debt Securities	$ 4