Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 3,194	$ 4,058
Gross unrealized gains	386	816
Gross unrealized losses	(43)	(31)
Total fair value	3,537	4,843
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	1,808	2,044
Gross unrealized gains	58	102
Gross unrealized losses	(38)	(27)
Total fair value	1,828	2,119
Other agency [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	256	332
Gross unrealized gains	16	25
Gross unrealized losses	0	0
Total fair value	272	357
Alt-A and subprime private-label securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	337	662
Gross unrealized gains	280	652
Gross unrealized losses	0	0
Total fair value	617	1,314
CMBS [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost		15
Gross unrealized gains		0
Gross unrealized losses		0
Total fair value		15
Mortgage revenue bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	454	655
Gross unrealized gains	15	20
Gross unrealized losses	(5)	(4)
Total fair value	464	671
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	339	350
Gross unrealized gains	17	17
Gross unrealized losses	0	0
Total fair value	$ 356	$ 367

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Section 50
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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+ References

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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