Investment Securities - Summary of Unrealized Loss Positions on Investment Securities (Details) $ in Millions	Sep. 30, 2020 USD ($) securities	Dec. 31, 2019 USD ($) securities
Agency - Commercial
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	$ 3	$ 148
Unrealized Loss Position with Duration Under 12 Months	$ 4	$ 303
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | securities	1	17
Unrealized Loss Position with Duration Under 12 Months | securities	1	19
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ 0	$ (3)
Unrealized Loss Position with Duration Under 12 Months	0	0
Fair Value
Unrealized Loss Position with Duration 12 Months and Over		148
Unrealized Loss Position with Duration Under 12 Months		$ 85
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | securities		13
Unrealized Loss Position with Duration Under 12 Months | securities		6
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over		$ (1)
Unrealized Loss Position with Duration Under 12 Months		0
Municipal obligations
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	0	8
Unrealized Loss Position with Duration Under 12 Months	$ 6	$ 0
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | securities	0	3
Unrealized Loss Position with Duration Under 12 Months | securities	1	0
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ 0	$ 0
Unrealized Loss Position with Duration Under 12 Months	0	0
Corporate debt obligations
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	0
Unrealized Loss Position with Duration Under 12 Months	$ 13
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | securities	0
Unrealized Loss Position with Duration Under 12 Months | securities	4
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ 0
Unrealized Loss Position with Duration Under 12 Months	0
Other mortgage-backed securities
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	0
Unrealized Loss Position with Duration Under 12 Months	$ 0
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | securities	0
Unrealized Loss Position with Duration Under 12 Months | securities	1
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ 0
Unrealized Loss Position with Duration Under 12 Months	0
Agency - Residential
Fair Value
Unrealized Loss Position with Duration 12 Months and Over		266
Unrealized Loss Position with Duration Under 12 Months		$ 148
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | securities		26
Unrealized Loss Position with Duration Under 12 Months | securities		14
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over		$ (3)
Unrealized Loss Position with Duration Under 12 Months		(1)
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	0	35
Unrealized Loss Position with Duration Under 12 Months	$ 5	$ 38
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | securities	0	7
Unrealized Loss Position with Duration Under 12 Months | securities	2	10
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ 0	$ (1)
Unrealized Loss Position with Duration Under 12 Months	$ 0	$ 0