Income Taxes (details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes disclosure
Current income tax expense included in consolidated statement of income, federal (excluding impact of TCJA)									$ 314
Current income tax expense included in consolidated statement of income, federal										$ 899	$ 1,144
Current income tax expense included in consolidated statement of income, impact of TCJA at enactment									21
Current income tax expense included in consolidated statement of income, foreign									56	21	29
Current income tax expense included in consolidated statement of income, state									4	8	9
Total current tax expense									395	928	1,182
Deferred income tax expense included in consolidated statement of income, federal (excluding impact of TCJA)									229
Deferred income tax expense included in consolidated statement of income, federal									337	110	117
Deferred income tax expense included in consolidated statement of income, impact of TCJA at enactment									108
Deferred income tax expense included in consolidated statement of income, foreign									(58)	1	2
Total deferred tax expense									279	111	119
Total income tax expense included in consolidated statement of income	$ 309	$ 27	$ 195	$ 143	$ 359	$ 231	$ 223	$ 226	674	1,039	1,301
Expense (benefit) relating to share-based compensation, the changes in unrealized gain on investments, unrealized loss on foreign exchange and other items in other comprehensive income (loss)									110	(289)	(448)
Total income tax expense included in consolidated financial statements									784	750	853
Net charge included in total income tax expense related to TCJA at the date of enactment	129								129
U.S. income before income taxes									2,798	3,946	4,621
Foreign income (loss) before income taxes									(68)	107	119
Income before income taxes	860	$ 320	$ 790	$ 760	1,302	$ 947	$ 887	$ 917	$ 2,730	$ 4,053	$ 4,740
Statutory tax rate (percent)									35.00%	35.00%	35.00%
Expected federal income tax expense									$ 956	$ 1,419	$ 1,659
Tax effect of nontaxable investment income									(297)	(323)	(345)
Tax effect of TCJA at enactment									129
Tax effect of other, net									$ (114)	$ (57)	$ (13)
Effective tax rate (percent)									25.00%	26.00%	27.00%
Income taxes paid									$ 514	$ 892	$ 1,207
Current income tax refundable	65								65
Current income tax payable					72					72
Estimated additional annual tax payments relating to the discounted reserves transitional adjustment associated with TCJA									70
Deferred tax assets, claims and claim adjustment expense reserves	930				664				930	664
Deferred tax assets, unearned premium reserves	478				760				478	760
Deferred tax assets, compensation-related liabilities	61				268				61	268
Deferred tax assets, other	191				272				191	272
Total gross deferred tax assets	1,660				1,964				1,660	1,964
Less: valuation allowance	6				3				6	3
Adjusted gross deferred tax assets	1,654				1,961				1,654	1,961
Deferred tax liabilities, claims and claim adjustment expense reserve discounting (transition rule)	560								560
Deferred tax liabilities, deferred acquisition costs	376				604				376	604
Deferred tax liabilities, investments	454				592				454	592
Deferred tax liabilities, internally-developed software	97				157				97	157
Deferred tax liabilities, other	97				143				97	143
Total gross deferred tax liabilities	1,584				1,496				1,584	1,496
Net deferred tax asset	$ 70				$ 465				70	$ 465
Increase in valuation allowance									3
Tax expense recognized resulting from from deemed repatriation of foreign earnings (part of net charge to record effect of TCJA)									$ 41