General - Statement of Operations (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Apr. 01, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 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	$ 6,356,000,000	$ 5,777,000,000	$ 12,176,000,000	$ 10,906,000,000
Operating costs and expenses	1,380,000,000	1,358,000,000	2,760,000,000	2,661,000,000
Pension and other postretirement remeasurement gains and losses	125,000,000	(7,000,000)	140,000,000	7,000,000
Restructuring and related charges	9,000,000	23,000,000	27,000,000	42,000,000
Total costs and expenses	10,068,000,000	9,204,000,000	19,439,000,000	17,712,000,000
Gain on disposition of operations	(2,000,000)	(2,000,000)	(3,000,000)	(3,000,000)
Income from operations before income tax expense	1,078,000,000	897,000,000	2,698,000,000	2,160,000,000
Income tax expense	227,000,000	180,000,000	555,000,000	437,000,000
Net income	851,000,000	717,000,000	2,143,000,000	1,723,000,000
Preferred stock dividends	30,000,000	39,000,000	61,000,000	68,000,000
Net income applicable to common shareholders	$ 821,000,000	$ 678,000,000	$ 2,082,000,000	$ 1,655,000,000
Net income available to common shareholders per common share - Basic (in dollars per share)	$ 2.47	$ 1.94	$ 6.27	$ 4.71
Net income available to common shareholders per common share - Diluted (in dollars per share)	$ 2.44	$ 1.91	$ 6.17	$ 4.63
Stockholders' equity	$ 24,476,000,000	$ 23,122,000,000	$ 24,476,000,000	$ 23,122,000,000			$ 21,312,000,000
Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense	6,376,000,000	5,792,000,000	12,205,000,000	10,941,000,000
Operating costs and expenses	1,401,000,000	1,384,000,000	2,789,000,000	2,717,000,000
Pension and other postretirement remeasurement gains and losses	0		0
Restructuring and related charges	5,000,000	27,000,000	27,000,000	49,000,000
Total costs and expenses	9,980,000,000	9,256,000,000	19,357,000,000	17,803,000,000
Income from operations before income tax expense	1,166,000,000	845,000,000	2,780,000,000	2,069,000,000
Income tax expense	246,000,000	169,000,000	573,000,000	418,000,000
Net income	920,000,000	676,000,000	2,207,000,000	1,651,000,000
Net income applicable to common shareholders	$ 890,000,000	$ 637,000,000	$ 2,146,000,000	$ 1,583,000,000
Net income available to common shareholders per common share - Basic (in dollars per share)	$ 2.68	$ 1.82	$ 6.46	$ 4.50
Net income available to common shareholders per common share - Diluted (in dollars per share)	$ 2.64	$ 1.80	$ 6.36	$ 4.43
Stockholders' equity	$ 24,703,000,000	$ 23,122,000,000	$ 24,703,000,000	$ 23,122,000,000
Impact of change | Impact of change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense	(20,000,000)	(15,000,000)	(29,000,000)	(35,000,000)
Operating costs and expenses	(21,000,000)	(26,000,000)	(29,000,000)	(56,000,000)
Pension and other postretirement remeasurement gains and losses	125,000,000		140,000,000
Restructuring and related charges	4,000,000	(4,000,000)	0	(7,000,000)
Total costs and expenses	88,000,000	(52,000,000)	82,000,000	(91,000,000)
Income from operations before income tax expense	(88,000,000)	52,000,000	(82,000,000)	91,000,000
Income tax expense	(19,000,000)	11,000,000	(18,000,000)	19,000,000
Net income	(69,000,000)	41,000,000	(64,000,000)	72,000,000
Net income applicable to common shareholders	$ (69,000,000)	$ 41,000,000	$ (64,000,000)	$ 72,000,000
Net income available to common shareholders per common share - Basic (in dollars per share)	$ (0.21)	$ 0.12	$ (0.19)	$ 0.21
Net income available to common shareholders per common share - Diluted (in dollars per share)	$ (0.20)	$ 0.11	$ (0.19)	$ 0.20
Stockholders' equity	$ (227,000,000)	$ 0	$ (227,000,000)	$ 0					$ 0
Retained income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	851,000,000	717,000,000	2,143,000,000	1,723,000,000
Stockholders' equity	45,803,000,000	43,997,000,000	45,803,000,000	43,997,000,000		$ 45,148,000,000	44,033,000,000	$ 43,479,000,000		$ 41,579,000,000
Retained income | Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	920,000,000	676,000,000	2,207,000,000	1,651,000,000
Stockholders' equity	47,542,000,000	45,508,000,000	47,542,000,000	45,508,000,000		46,818,000,000	45,708,000,000	45,031,000,000		43,162,000,000
Retained income | Impact of change | Impact of change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	(69,000,000)	41,000,000	(64,000,000)	72,000,000
Stockholders' equity	(1,739,000,000)	(1,511,000,000)	(1,739,000,000)	(1,511,000,000)	$ (18,000,000)	(1,670,000,000)	(1,675,000,000)	(1,552,000,000)	(1,580,000,000)	(1,583,000,000)
Accumulated other comprehensive income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	1,760,000,000	243,000,000	1,760,000,000	243,000,000		1,085,000,000	118,000,000	398,000,000		1,889,000,000
Accumulated other comprehensive income | Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	248,000,000	(1,268,000,000)	248,000,000	(1,268,000,000)		(566,000,000)	(1,557,000,000)	(1,150,000,000)		306,000,000
Accumulated other comprehensive income | Impact of change | Impact of change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	$ 1,512,000,000	$ 1,511,000,000	$ 1,512,000,000	$ 1,511,000,000	$ 18,000,000	$ 1,651,000,000	$ 1,675,000,000	$ 1,548,000,000	$ 1,580,000,000	$ 1,583,000,000