Summary of Significant Accounting Policies - Consolidated Statement of Operations (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense									$ 23,976,000,000	$ 22,778,000,000	$ 21,847,000,000
Operating costs and expenses									5,690,000,000	5,594,000,000	5,196,000,000
Pension and other postretirement remeasurement gains and losses									114,000,000	468,000,000	(217,000,000)
Restructuring and related charges									41,000,000	67,000,000	96,000,000
Total costs and expenses									38,592,000,000	37,193,000,000	34,878,000,000
Total costs and expenses									6,089,000,000	2,628,000,000	4,549,000,000
Income tax expense									1,242,000,000	468,000,000	995,000,000
Net income									4,847,000,000	2,160,000,000	3,554,000,000
Net Income (Loss) Available to Common Stockholders, Basic	$ 1,707,000,000	$ 889,000,000	$ 821,000,000	$ 1,261,000,000	$ (585,000,000)	$ 942,000,000	$ 678,000,000	$ 977,000,000	$ 4,678,000,000	$ 2,012,000,000	$ 3,438,000,000
Net income applicable to common shareholders earnings per common share - Basic (in dollars per share)	$ 5.32	$ 2.71	$ 2.47	$ 3.79	$ (1.71)	$ 2.72	$ 1.94	$ 2.76	$ 14.25	$ 5.78	$ 9.50
Net income applicable to common shareholders earnings per common share - Diluted (in dollars per share)	$ 5.23	$ 2.67	$ 2.44	$ 3.74	$ (1.71)	$ 2.68	$ 1.91	$ 2.71	$ 14.03	$ 5.70	$ 9.35
Impact of merger	$ 25,998,000,000				$ 21,312,000,000				$ 25,998,000,000	$ 21,312,000,000	$ 22,551,000,000
Change in accounting principle
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income (Loss) Available to Common Stockholders, Basic	$ 240,000,000	$ (140,000,000)	$ (69,000,000)	$ 5,000,000	$ (273,000,000)	$ 109,000,000	$ 41,000,000	$ 31,000,000
Net income applicable to common shareholders earnings per common share - Basic (in dollars per share)	$ 0.75	$ (0.43)	$ (0.21)	$ 0.01	$ (0.80)	$ 0.31	$ 0.12	$ 0.09
Net income applicable to common shareholders earnings per common share - Diluted (in dollars per share)	$ 0.73	$ (0.42)	$ (0.20)	$ 0.02	$ (0.80)	$ 0.31	$ 0.11	$ 0.08
Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense									24,074,000,000	22,839,000,000	21,929,000,000
Operating costs and expenses									5,752,000,000	5,869,000,000	5,442,000,000
Pension and other postretirement remeasurement gains and losses									0	0	0
Restructuring and related charges									41,000,000	83,000,000	109,000,000
Total costs and expenses									38,638,000,000	37,077,000,000	35,436,000,000
Total costs and expenses									6,043,000,000	2,744,000,000	3,991,000,000
Income tax expense									1,232,000,000	492,000,000	802,000,000
Net income									4,811,000,000	2,252,000,000	3,189,000,000
Net Income (Loss) Available to Common Stockholders, Basic									$ 4,642,000,000	$ 2,104,000,000	$ 3,073,000,000
Net income applicable to common shareholders earnings per common share - Basic (in dollars per share)									$ 14.14	$ 6.05	$ 8.49
Net income applicable to common shareholders earnings per common share - Diluted (in dollars per share)									$ 13.92	$ 5.96	$ 8.36
Impact of merger	$ 26,156,000,000				$ 21,312,000,000				$ 26,156,000,000	$ 21,312,000,000
Change in accounting principle | Change in accounting principle
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense									(98,000,000)	(61,000,000)	$ (82,000,000)
Operating costs and expenses									(62,000,000)	(275,000,000)	(246,000,000)
Pension and other postretirement remeasurement gains and losses									114,000,000	468,000,000	(217,000,000)
Restructuring and related charges									0	(16,000,000)	(13,000,000)
Total costs and expenses									(46,000,000)	116,000,000	(558,000,000)
Total costs and expenses									46,000,000	(116,000,000)	558,000,000
Income tax expense									10,000,000	(24,000,000)	193,000,000
Net income									36,000,000	(92,000,000)	365,000,000
Net Income (Loss) Available to Common Stockholders, Basic									$ 36,000,000	$ (92,000,000)	$ 365,000,000
Net income applicable to common shareholders earnings per common share - Basic (in dollars per share)									$ 0.11	$ (0.27)	$ 1.01
Net income applicable to common shareholders earnings per common share - Diluted (in dollars per share)									$ 0.11	$ (0.26)	$ 0.99
Impact of merger	$ (158,000,000)				$ 0				$ (158,000,000)	$ 0		$ (41,000,000)	$ 0