Investments (Securities in a gross unrealized loss position by ratings) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	$ 27,012	$ 25,731
Gross Unrealized Losses	3,096	2,995
U.S. Government, Government agencies and Government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,426	2,273
Gross Unrealized Losses	361	309
AAA
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,661	1,524
Gross Unrealized Losses	280	261
AA
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	4,109	3,817
Gross Unrealized Losses	690	658
A
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	6,324	5,652
Gross Unrealized Losses	570	517
BBB
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	11,672	11,523
Gross Unrealized Losses	1,083	1,095
Non-investment grade
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	820	942
Gross Unrealized Losses	$ 112	$ 155