Segment Information (Financial Data By Reportable Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Reportable segments operating revenues	$ 25,177	$ 22,715	$ 49,565	$ 45,057
Operating gain (loss)	1,433	1,559	3,373	3,427
Commercial & Specialty [Member]
Segment Reporting Information [Line Items]
Operating gain (loss)	983	1,048	2,581	2,450
Government Business [Member]
Segment Reporting Information [Line Items]
Operating gain (loss)	478	532	855	1,021
Other [Member]
Segment Reporting Information [Line Items]
Operating gain (loss)	(28)	(21)	(63)	(44)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Reportable segments operating revenues	(572)	(309)	(1,050)	(631)
Unaffiliated
Segment Reporting Information [Line Items]
Reportable segments operating revenues	25,177	22,715	49,565	45,057
Unaffiliated | Commercial & Specialty [Member]
Segment Reporting Information [Line Items]
Reportable segments operating revenues	9,417	9,055	18,809	18,006
Unaffiliated | Government Business [Member]
Segment Reporting Information [Line Items]
Reportable segments operating revenues	15,538	13,619	30,464	26,972
Unaffiliated | Other [Member]
Segment Reporting Information [Line Items]
Reportable segments operating revenues	222	41	292	79
Affiliated [Member]
Segment Reporting Information [Line Items]
Reportable segments operating revenues	0
Affiliated [Member] | Commercial & Specialty [Member]
Segment Reporting Information [Line Items]
Reportable segments operating revenues	0	0	0	0
Affiliated [Member] | Government Business [Member]
Segment Reporting Information [Line Items]
Reportable segments operating revenues	0	0	0	0
Affiliated [Member] | Other [Member]
Segment Reporting Information [Line Items]
Reportable segments operating revenues	$ 572	$ 309	$ 1,050	$ 631