Investments in Securities Available-for-sale Securities 3 (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	$ 2	$ 1
Less than 12 Consecutive Months, Fair Value	88	130
12 Consecutive Months or Longer, Gross Unrealized Losses	(10)	(13)
12 Consecutive Months or Longer, Fair Value	105	340
AOCI, Debt Securities, Available-for-sale without Allowance for Credit Loss, Cumulative Gain (Loss), after Tax	115
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	105	340
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	88	130
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	10	13
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax	(31)	(34)
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	1	0
Less than 12 Consecutive Months, Fair Value	66	0
12 Consecutive Months or Longer, Gross Unrealized Losses	(10)	(10)
12 Consecutive Months or Longer, Fair Value	105	337
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	105	337
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	66	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	10	10
Mortgage revenue bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	1	0
Less than 12 Consecutive Months, Fair Value	22	0
12 Consecutive Months or Longer, Gross Unrealized Losses	0	(3)
12 Consecutive Months or Longer, Fair Value	0	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	22	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0	3
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses		1
Less than 12 Consecutive Months, Fair Value		130
12 Consecutive Months or Longer, Gross Unrealized Losses		0
12 Consecutive Months or Longer, Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		130
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		$ 0