Investment Securities - Summary (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022
Available-for-sale debt securities:
Cost		$ 2,138
Estimated Fair Value		2,120	$ 4,537
Equity securities
Cost		60	50
Gross Unrealized Gains		16	0
Gross Unrealized Losses		(10)	(9)
Estimated Fair Value		66	41
Total Cost		2,198	4,671
Total Gross Unrealized Gains		17	0
Total Gross Unrealized Losses		(29)	(93)
Total Estimated Fair Value		2,186	4,578
New public companies
Equity securities
Cost		10
Estimated Fair Value		24
Net unrealized gains on equity securities	$ 37
State and municipal obligations
Available-for-sale debt securities:
Cost		61	64
Gross Unrealized Gains		0	0
Gross Unrealized Losses		(6)	(10)
Estimated Fair Value		55	54
U.S. Government agency obligations
Available-for-sale debt securities:
Cost		4	5
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Estimated Fair Value		4	5
U.S. Government treasury obligations
Available-for-sale debt securities:
Cost		1,217	3,859
Gross Unrealized Gains		1	0
Gross Unrealized Losses		(12)	(73)
Estimated Fair Value		1,206	3,786
Mortgage-backed securities
Available-for-sale debt securities:
Cost		12	13
Gross Unrealized Gains		0	0
Gross Unrealized Losses		(1)	0
Estimated Fair Value		11	13
Foreign government bonds and obligations
Available-for-sale debt securities:
Cost		770	633
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	(1)
Estimated Fair Value		770	632
Other
Available-for-sale debt securities:
Cost		74	47
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Estimated Fair Value		$ 74	$ 47