Other Comprehensive Income (Loss) - Components of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (676)	$ (1,646)	$ (65)
Other comprehensive income (loss) before reclassifications	(125)	570	(1,828)
Amounts reclassified from accumulated other comprehensive income	81	400	247
Net current period other comprehensive income (loss)	(44)	970	(1,581)
Ending balance	(720)	(676)	(1,646)
Unrealized Appreciation on Available-For-Sale Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(565)	(1,512)	13
Other comprehensive income (loss) before reclassifications	(23)	547	(1,772)
Amounts reclassified from accumulated other comprehensive income	81	400	247
Net current period other comprehensive income (loss)	58	947	(1,525)
Ending balance	(507)	(565)	(1,512)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(111)	(134)	(78)
Other comprehensive income (loss) before reclassifications	(102)	23	(56)
Amounts reclassified from accumulated other comprehensive income	0	0	0
Net current period other comprehensive income (loss)	(102)	23	(56)
Ending balance	$ (213)	$ (111)	$ (134)