Casualty, Environmental and Other Reserves - Schedule of Claims Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 314	$ 305	$ 324
Assumed in Acquisition of Quality Carriers	62
Charged to Expense	130	107	107
Payments	(138)	(98)	(126)
Balance end of period	368	314	305
Casualty Reserves
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	196	187	199
Assumed in Acquisition of Quality Carriers	0
Charged to Expense	55	55	56
Payments	(71)	(46)	(68)
Balance end of period	180	196	187
Environmental Reserves
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	76	74	80
Assumed in Acquisition of Quality Carriers	29
Charged to Expense	26	20	17
Payments	(23)	(18)	(23)
Balance end of period	108	76	74
Other Reserves
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	42	44	45
Assumed in Acquisition of Quality Carriers	33
Charged to Expense	49	32	34
Payments	(44)	(34)	(35)
Balance end of period	$ 80	$ 42	$ 44