General - Statement of Operations (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jul. 01, 2019	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense	$ 6,051,000,000	$ 5,805,000,000	$ 18,227,000,000	$ 16,711,000,000
Operating costs and expenses	1,414,000,000	1,425,000,000	4,174,000,000	4,086,000,000
Pension and other postretirement remeasurement gains and losses	(225,000,000)	39,000,000	(365,000,000)	32,000,000
Restructuring and related charges	0	13,000,000	27,000,000	55,000,000
Total costs and expenses	9,909,000,000	9,288,000,000	29,348,000,000	27,000,000,000
Gain on disposition of operations	0	(1,000,000)	(3,000,000)	(4,000,000)
Income from operations before income tax expense	1,160,000,000	1,178,000,000	3,858,000,000	3,338,000,000
Income tax expense	229,000,000	199,000,000	784,000,000	636,000,000
Net income	931,000,000	979,000,000	3,074,000,000	2,702,000,000
Net income applicable to common shareholders	$ 889,000,000	$ 942,000,000	$ 2,971,000,000	$ 2,597,000,000
Net income available to common shareholders per common share - Basic (in dollars per share)	$ 2.71	$ 2.72	$ 8.98	$ 7.43
Net income available to common shareholders per common share - Diluted (in dollars per share)	$ 2.67	$ 2.68	$ 8.85	$ 7.31
Total shareholders’ equity	$ 26,140,000,000	$ 23,659,000,000	$ 26,140,000,000	$ 23,659,000,000				$ 21,312,000,000
Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense	6,080,000,000	5,817,000,000	18,285,000,000	16,758,000,000
Operating costs and expenses	1,433,000,000	1,510,000,000	4,222,000,000	4,227,000,000
Pension and other postretirement remeasurement gains and losses	0	0	0	0
Restructuring and related charges	0	16,000,000	27,000,000	65,000,000
Total costs and expenses	9,732,000,000	9,427,000,000	29,089,000,000	27,230,000,000
Income from operations before income tax expense	1,337,000,000	1,039,000,000	4,117,000,000	3,108,000,000
Income tax expense	266,000,000	169,000,000	839,000,000	587,000,000
Net income	1,071,000,000	870,000,000	3,278,000,000	2,521,000,000
Net income applicable to common shareholders	$ 1,029,000,000	$ 833,000,000	$ 3,175,000,000	$ 2,416,000,000
Net income available to common shareholders per common share - Basic (in dollars per share)	$ 3.14	$ 2.41	$ 9.60	$ 6.91
Net income available to common shareholders per common share - Diluted (in dollars per share)	$ 3.09	$ 2.37	$ 9.46	$ 6.80
Total shareholders’ equity	$ 26,537,000,000	$ 23,633,000,000	$ 26,537,000,000	$ 23,633,000,000
Impact of change | Impact of change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense	(29,000,000)	(12,000,000)	(58,000,000)	(47,000,000)
Operating costs and expenses	(19,000,000)	(85,000,000)	(48,000,000)	(141,000,000)
Pension and other postretirement remeasurement gains and losses	(225,000,000)	39,000,000	(365,000,000)	32,000,000
Restructuring and related charges	0	(3,000,000)	0	(10,000,000)
Total costs and expenses	177,000,000	(139,000,000)	259,000,000	(230,000,000)
Income from operations before income tax expense	(177,000,000)	139,000,000	(259,000,000)	230,000,000
Income tax expense	(37,000,000)	30,000,000	(55,000,000)	49,000,000
Net income	(140,000,000)	109,000,000	(204,000,000)	181,000,000
Net income applicable to common shareholders	$ (140,000,000)	$ 109,000,000	$ (204,000,000)	$ 181,000,000
Net income available to common shareholders per common share - Basic (in dollars per share)	$ (0.43)	$ 0.31	$ (0.62)	$ 0.52
Net income available to common shareholders per common share - Diluted (in dollars per share)	$ (0.42)	$ 0.31	$ (0.61)	$ 0.51
Total shareholders’ equity	$ (397,000,000)	$ 26,000,000	$ (397,000,000)	$ 26,000,000	$ (12,000,000)		$ (18,000,000)			$ 0
Retained income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	931,000,000	979,000,000	3,074,000,000	2,702,000,000
Total shareholders’ equity	46,527,000,000	44,776,000,000	46,527,000,000	44,776,000,000		$ 45,803,000,000		44,033,000,000	$ 43,997,000,000		$ 41,579,000,000
Retained income | Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	1,071,000,000	870,000,000	3,278,000,000	2,521,000,000
Total shareholders’ equity	48,406,000,000	46,178,000,000	48,406,000,000	46,178,000,000		47,542,000,000		45,708,000,000	45,508,000,000		43,162,000,000
Retained income | Impact of change | Impact of change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	(140,000,000)	109,000,000	(204,000,000)	181,000,000
Total shareholders’ equity	$ (1,879,000,000)	$ (1,402,000,000)	$ (1,879,000,000)	$ (1,402,000,000)		$ (1,739,000,000)		$ (1,675,000,000)	$ (1,511,000,000)	$ (1,580,000,000)	$ (1,583,000,000)