Schedule II-Condensed Financial Information Of Registrant (Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 5,741	$ 4,937	$ 3,934	$ 3,609
Fixed maturity securities, current (amortized cost of $594 and $592; allowance for credit losses of $0 and $0)	23,433	19,676
Equity securities	1,559	1,009
Other receivables	2,830	2,807
Other current assets	4,060	3,020
Total current assets	45,751	39,033
Fixed maturity securities (Parent Company - amortized cost $0)	562	505
Other invested assets, long-term	4,285	4,258
Property and equipment, net	3,483	3,133
Other noncurrent assets	1,438	1,350
Total assets	86,615	77,453
Current liabilities:
Accounts payable and accrued expenses	5,493	4,198
Current portion of long-term debt	700	1,598
Other current liabilities	6,052	4,127
Total current liabilities	29,453	23,532
Long-term debt, less current portion	19,335	17,787
Other noncurrent liabilities	1,815	1,420
Total liabilities	53,416	45,725
Commitments and contingencies-Note 5
Shareholders' equity
Preferred stock, without par value, shares authorized - 100,000,000; shares issued and outstanding - none	0	0
Common stock, par value $0.01, shares authorized - 900,000,000; shares issued and outstanding - 245,401,430 and 252,922,161	3	3
Additional paid-in capital	9,244	9,448
Retained earnings	23,802	22,573
Accumulated other comprehensive income (loss)	150	(296)
Total shareholders' equity	33,199	31,728	28,541	26,503
Total liabilities and shareholders' equity	86,615	77,453
Parent Company [Member]
Current assets:
Cash and cash equivalents	700	1,818	$ 1,290	$ 956
Fixed maturity securities, current (amortized cost of $594 and $592; allowance for credit losses of $0 and $0)	608	602
Equity securities	439	253
Other receivables	41	92
Net due from subsidiaries	0	602
Other current assets	800	653
Total current assets	2,588	4,020
Other invested assets, long-term	664	657
Property and equipment, net	209	170
Deferred tax assets, net	391	216
Investment in subsidiaries	51,739	47,423
Other noncurrent assets	211	263
Total assets	55,802	52,749
Current liabilities:
Accounts payable and accrued expenses	429	887
Net due to subsidiaries	1,239	0
Current portion of long-term debt	700	1,598
Other current liabilities	494	263
Total current liabilities	2,862	2,748
Long-term debt, less current portion	19,310	17,762
Other noncurrent liabilities	431	511
Total liabilities	22,603	21,021
Commitments and contingencies-Note 5
Shareholders' equity
Preferred stock, without par value, shares authorized - 100,000,000; shares issued and outstanding - none	0	0
Common stock, par value $0.01, shares authorized - 900,000,000; shares issued and outstanding - 245,401,430 and 252,922,161	3	3
Additional paid-in capital	9,244	9,448
Retained earnings	23,802	22,573
Accumulated other comprehensive income (loss)	150	(296)
Total shareholders' equity	33,199	31,728
Total liabilities and shareholders' equity	$ 55,802	$ 52,749