Income Taxes (details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes disclosure
Current income tax expense included in consolidated statement of income, federal									$ 1,216	$ 1,059	$ 406
Current income tax expense included in consolidated statement of income, foreign									28	30	45
Current income tax expense included in consolidated statement of income, state									10	6	3
Total current tax expense									1,254	1,095	454
Deferred income tax expense included in consolidated statement of income, federal									121	167	223
Deferred income tax expense included in consolidated statement of income, foreign									22	10	16
Total deferred tax expense									143	177	239
Total income tax expense included in consolidated statement of income	422	339	218	418	383	313	252	324	1,397	1,272	693
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									68	(822)	57
Total income tax expense included in consolidated financial statements									1,465	450	750
U.S. income before income taxes									4,899	4,804	2,955
Foreign income before income taxes									190	141	211
Income before income taxes	1,460	1,258	901	1,470	1,371	1,177	1,177	1,220	5,089	4,945	3,166
Statutory tax rate (percent)									35.00%	35.00%	35.00%
Expected federal income tax expense									1,781	1,731	1,108
Tax effect of nontaxable investment income									(379)	(409)	(427)
Tax effect of other, net									(5)	(50)	12
Effective tax rate (percent)									27.00%	26.00%	22.00%
Income taxes paid									1,147	1,057	188
Current income tax payable	139				85				139	85
Deferred tax assets, claims and claim adjustment expense reserves	768				825				768	825
Deferred tax assets, unearned premium reserves	709				693				709	693
Deferred tax assets, compensation-related liabilities	345				207				345	207
Deferred tax assets, other	346				356				346	356
Total gross deferred tax assets	2,168				2,081				2,168	2,081
Deferred tax liabilities, deferred acquisition costs	565				554				565	554
Deferred tax liabilities, investments	1,267				931				1,267	931
Deferred tax liabilities, internally-developed software	130				138				130	138
Deferred tax liabilities, other	173				155				173	155
Total gross deferred tax liabilities	2,135				1,778				2,135	1,778
Net deferred tax asset	33				303				33	303
Amount of the Company's foreign operations' undistributed earnings for which U.S. income taxes have not been recognized	$ 647								$ 647