Casualty, Environmental and Other Reserves - Schedule of Claims Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 436	$ 368	$ 314
Charged to Expense	165	143	130
Payments	(161)	(130)	(138)
Balance end of period	440	436	368
Quality Carriers, Inc.
Loss Contingency Accrual [Roll Forward]
Assumed in Acquisition			62
Pan-Am
Loss Contingency Accrual [Roll Forward]
Assumed in Acquisition		55
Casualty Reserves
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	194	180	196
Charged to Expense	69	45	55
Payments	(68)	(50)	(71)
Balance end of period	195	194	180
Casualty Reserves | Quality Carriers, Inc.
Loss Contingency Accrual [Roll Forward]
Assumed in Acquisition			0
Casualty Reserves | Pan-Am
Loss Contingency Accrual [Roll Forward]
Assumed in Acquisition		19
Environmental Reserves
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	161	108	76
Charged to Expense	29	47	26
Payments	(36)	(30)	(23)
Balance end of period	154	161	108
Environmental Reserves | Quality Carriers, Inc.
Loss Contingency Accrual [Roll Forward]
Assumed in Acquisition			29
Environmental Reserves | Pan-Am
Loss Contingency Accrual [Roll Forward]
Assumed in Acquisition		36
Other Reserves
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	81	80	42
Charged to Expense	67	51	49
Payments	(57)	(50)	(44)
Balance end of period	$ 91	81	80
Other Reserves | Quality Carriers, Inc.
Loss Contingency Accrual [Roll Forward]
Assumed in Acquisition			$ 33
Other Reserves | Pan-Am
Loss Contingency Accrual [Roll Forward]
Assumed in Acquisition		$ 0