Investment Securities - Securities in Gross Unrealized Loss Position (Detail) $ in Millions	Sep. 30, 2020 USD ($) Security	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 8,053	$ 15,882
Fair Value - 12 Months or Longer	2,126	12,252
Fair Value - Total	10,179	28,134
Gross Unrealized Loss - Less than 12 Months	(55)	(120)
Gross Unrealized Loss - 12 Months or Longer	(41)	(212)
Gross Unrealized Loss - Total	$ (96)	(332)
Number of securities in gross unrealized loss positions \| Security	300
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 0	2,647
Fair Value - 12 Months or Longer	0	0
Fair Value - Total	0	2,647
Gross Unrealized Loss - Less than 12 Months	0	(4)
Gross Unrealized Loss - 12 Months or Longer	0	0
Gross Unrealized Loss - Total	0	(4)
RMBS, Agency
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	6,681	10,494
Fair Value - 12 Months or Longer	1,866	10,567
Fair Value - Total	8,547	21,061
Gross Unrealized Loss - Less than 12 Months	(51)	(92)
Gross Unrealized Loss - 12 Months or Longer	(37)	(192)
Gross Unrealized Loss - Total	(88)	(284)
RMBS, Non-agency
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	14	35
Fair Value - 12 Months or Longer	0	16
Fair Value - Total	14	51
Gross Unrealized Loss - Less than 12 Months	0	(1)
Gross Unrealized Loss - 12 Months or Longer	0	(1)
Gross Unrealized Loss - Total	0	(2)
RMBS
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	6,695	10,529
Fair Value - 12 Months or Longer	1,866	10,583
Fair Value - Total	8,561	21,112
Gross Unrealized Loss - Less than 12 Months	(51)	(93)
Gross Unrealized Loss - 12 Months or Longer	(37)	(193)
Gross Unrealized Loss - Total	(88)	(286)
CMBS, Agency
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	916	2,580
Fair Value - 12 Months or Longer	257	1,563
Fair Value - Total	1,173	4,143
Gross Unrealized Loss - Less than 12 Months	(4)	(23)
Gross Unrealized Loss - 12 Months or Longer	(4)	(19)
Gross Unrealized Loss - Total	(8)	(42)
Other securities
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	442	126
Fair Value - 12 Months or Longer	3	106
Fair Value - Total	445	232
Gross Unrealized Loss - Less than 12 Months	0	0
Gross Unrealized Loss - 12 Months or Longer	0	0
Gross Unrealized Loss - Total	$ 0	$ 0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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