Income Taxes (details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes disclosure
Current income tax expense included in consolidated statement of income, federal									$ 1,144	$ 1,216	$ 1,059
Current income tax expense included in consolidated statement of income, foreign									29	28	30
Current income tax expense included in consolidated statement of income, state									9	10	6
Total current tax expense									1,182	1,254	1,095
Deferred income tax expense included in consolidated statement of income, federal									117	121	167
Deferred income tax expense included in consolidated statement of income, foreign									2	22	10
Total deferred tax expense									119	143	177
Total income tax expense included in consolidated statement of income	$ 343	$ 379	$ 264	$ 315	$ 422	$ 339	$ 218	$ 418	1,301	1,397	1,272
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									(448)	68	(822)
Total income tax expense included in consolidated financial statements									853	1,465	450
U.S. income before income taxes									4,621	4,899	4,804
Foreign income before income taxes									119	190	141
Income before income taxes	1,209	$ 1,307	$ 1,076	$ 1,148	1,460	$ 1,258	$ 901	$ 1,470	$ 4,740	$ 5,089	$ 4,945
Statutory tax rate (percent)									35.00%	35.00%	35.00%
Expected federal income tax expense									$ 1,659	$ 1,781	$ 1,731
Tax effect of nontaxable investment income									(345)	(379)	(409)
Tax effect of other, net									$ (13)	$ (5)	$ (50)
Effective tax rate (percent)									27.00%	27.00%	26.00%
Income taxes paid									$ 1,207	$ 1,147	$ 1,057
Current income tax payable	50				139				50	139
Deferred tax assets, claims and claim adjustment expense reserves	691				768				691	768
Deferred tax assets, unearned premium reserves	731				709				731	709
Deferred tax assets, compensation-related liabilities	326				345				326	345
Deferred tax assets, other	320				346				320	346
Total gross deferred tax assets	2,068				2,168				2,068	2,168
Deferred tax liabilities, deferred acquisition costs	580				565				580	565
Deferred tax liabilities, investments	867				1,267				867	1,267
Deferred tax liabilities, internally-developed software	134				130				134	130
Deferred tax liabilities, other	191				173				191	173
Total gross deferred tax liabilities	1,772				2,135				1,772	2,135
Net deferred tax asset	296				$ 33				296	$ 33
Amount of the Company's foreign operations' undistributed earnings for which U.S. income taxes have not been recognized	$ 383								$ 383