Investment Securities - Securities in Gross Unrealized Loss Position (Details) $ in Millions	Mar. 31, 2024 USD ($) security	Dec. 31, 2023 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 4,603	$ 5,068
Gross Unrealized Loss - Less than 12 Months	(42)	(50)
Fair Value - 12 Months or Longer	62,633	62,711
Gross Unrealized Loss - 12 Months or Longer	(10,197)	(9,102)
Fair Value - Total	67,236	67,779
Gross Unrealized Loss - Total	$ (10,239)	$ (9,152)
Number of securities in gross unrealized loss positions | security	2,770	2,740
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 443	$ 733
Gross Unrealized Loss - Less than 12 Months	(1)	0
Fair Value - 12 Months or Longer	2,099	2,242
Gross Unrealized Loss - 12 Months or Longer	(38)	(49)
Fair Value - Total	2,542	2,975
Gross Unrealized Loss - Total	(39)	(49)
RMBS, Agency
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	3,213	3,511
Gross Unrealized Loss - Less than 12 Months	(36)	(43)
Fair Value - 12 Months or Longer	53,912	53,987
Gross Unrealized Loss - 12 Months or Longer	(9,470)	(8,407)
Fair Value - Total	57,125	57,498
Gross Unrealized Loss - Total	(9,506)	(8,450)
RMBS, Non-agency
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	5	1
Gross Unrealized Loss - Less than 12 Months	0	0
Fair Value - 12 Months or Longer	14	13
Gross Unrealized Loss - 12 Months or Longer	(1)	(1)
Fair Value - Total	19	14
Gross Unrealized Loss - Total	(1)	(1)
Total RMBS
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	3,218	3,512
Gross Unrealized Loss - Less than 12 Months	(36)	(43)
Fair Value - 12 Months or Longer	53,926	54,000
Gross Unrealized Loss - 12 Months or Longer	(9,471)	(8,408)
Fair Value - Total	57,144	57,512
Gross Unrealized Loss - Total	(9,507)	(8,451)
Agency CMBS
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	544	547
Gross Unrealized Loss - Less than 12 Months	(5)	(7)
Fair Value - 12 Months or Longer	6,509	6,465
Gross Unrealized Loss - 12 Months or Longer	(688)	(645)
Fair Value - Total	7,053	7,012
Gross Unrealized Loss - Total	(693)	(652)
Other securities
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	398	276
Gross Unrealized Loss - Less than 12 Months	0	0
Fair Value - 12 Months or Longer	99	4
Gross Unrealized Loss - 12 Months or Longer	0	0
Fair Value - Total	497	280
Gross Unrealized Loss - Total	$ 0	$ 0