General - Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense	$ 5,820	$ 5,129
Operating costs and expenses	1,380	1,303
Pension and other postretirement remeasurement gains and losses	15	14
Restructuring and related charges	18	19
Total costs and expenses	9,371	8,508
Gain on disposition of operations	(1)	(1)
Income from operations before income tax expense	1,620	1,263
Income tax expense	328	257
Net income	1,292	1,006
Preferred stock dividends	31	29
Net income applicable to common shareholders	$ 1,261	$ 977
Net income available to common shareholders per common share - Basic (in dollars per share)	$ 3.79	$ 2.76
Net income available to common shareholders per common share - Diluted (in dollars per share)	$ 3.74	$ 2.71
Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense	$ 5,829	$ 5,149
Operating costs and expenses	1,388	1,333
Pension and other postretirement remeasurement gains and losses	0	0
Restructuring and related charges	22	22
Total costs and expenses	9,377	8,547
Income from operations before income tax expense	1,614	1,224
Income tax expense	327	249
Net income	1,287	975
Net income applicable to common shareholders	$ 1,256	$ 946
Net income available to common shareholders per common share - Basic (in dollars per share)	$ 3.78	$ 2.67
Net income available to common shareholders per common share - Diluted (in dollars per share)	$ 3.72	$ 2.63
Impact of change | Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and casualty insurance claims and claims expense	$ (9)	$ (20)
Operating costs and expenses	(8)	(30)
Pension and other postretirement remeasurement gains and losses	(15)	14
Restructuring and related charges	(4)	(3)
Total costs and expenses	(6)	(39)
Income from operations before income tax expense	6	39
Income tax expense	1	8
Net income	5	31
Net income applicable to common shareholders	$ 5	$ 31
Net income available to common shareholders per common share - Basic (in dollars per share)	$ 0.01	$ 0.09
Net income available to common shareholders per common share - Diluted (in dollars per share)	$ 0.02	$ 0.08