Investment Securities - Summary of Unrealized Loss Positions on Investment Securities (Details) $ in Millions	Mar. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Other MBS
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	$ 0
Unrealized Loss Position with Duration Under 12 Months	$ 16
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | security	0
Unrealized Loss Position with Duration Under 12 Months | security	2
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ 0
Unrealized Loss Position with Duration Under 12 Months	0
Corporate debt obligations
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	0	$ 0
Unrealized Loss Position with Duration Under 12 Months	$ 11	$ 7
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | security	0	0
Unrealized Loss Position with Duration Under 12 Months | security	3	2
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ 0	$ 0
Unrealized Loss Position with Duration Under 12 Months	0	0
Municipal obligations
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	27	28
Unrealized Loss Position with Duration Under 12 Months	$ 0	$ 1
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | security	14	16
Unrealized Loss Position with Duration Under 12 Months | security	0	2
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ 0	$ (1)
Unrealized Loss Position with Duration Under 12 Months	0	0
Agency - Residential
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	624	647
Unrealized Loss Position with Duration Under 12 Months	$ 0	$ 14
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | security	78	79
Unrealized Loss Position with Duration Under 12 Months | security	0	5
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ (15)	$ (24)
Unrealized Loss Position with Duration Under 12 Months	0	0
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	306	345
Unrealized Loss Position with Duration Under 12 Months	$ 0	$ 0
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | security	55	60
Unrealized Loss Position with Duration Under 12 Months | security	0	0
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ (4)	$ (9)
Unrealized Loss Position with Duration Under 12 Months	0	0
Agency - Commercial
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	1,011	1,025
Unrealized Loss Position with Duration Under 12 Months	$ 21	$ 1
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | security	73	74
Unrealized Loss Position with Duration Under 12 Months | security	3	1
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ (34)	$ (43)
Unrealized Loss Position with Duration Under 12 Months	0	0
Fair Value
Unrealized Loss Position with Duration 12 Months and Over	326	336
Unrealized Loss Position with Duration Under 12 Months	$ 0	$ 0
Number of Securities
Unrealized Loss Position with Duration 12 Months and Over | security	26	26
Unrealized Loss Position with Duration Under 12 Months | security	0	0
Unrealized Loss
Unrealized Loss Position with Duration 12 Months and Over	$ (13)	$ (13)
Unrealized Loss Position with Duration Under 12 Months	$ 0	$ 0