Segment Information Segment Information (Major Product Revenues for Each Reportable Segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from External Customer [Line Items]
Insurance Services Revenue	$ 22,980	$ 22,097	$ 68,037	$ 66,614
Other Segment [Member]
Revenue from External Customer [Line Items]
Insurance Services Revenue	11	8	33	17
Commercial Specialty Business Segment [Member]
Revenue from External Customer [Line Items]
Insurance Services Revenue	9,128	10,052	27,357	30,651
Government Business Segment [Member]
Revenue from External Customer [Line Items]
Insurance Services Revenue	13,841	12,037	40,647	35,946
Managed Care Products [Member] | Commercial Specialty Business Segment [Member]
Revenue from External Customer [Line Items]
Insurance Services Revenue	7,377	8,478	22,212	25,815
Managed Care Products [Member] | Government Business Segment [Member]
Revenue from External Customer [Line Items]
Insurance Services Revenue	13,695	11,934	40,276	35,619
Managed Care Services [Member] | Commercial Specialty Business Segment [Member]
Revenue from External Customer [Line Items]
Insurance Services Revenue	1,334	1,196	3,918	3,651
Managed Care Services [Member] | Government Business Segment [Member]
Revenue from External Customer [Line Items]
Insurance Services Revenue	146	103	371	327
Dental Vision Products And Services [Member] | Commercial Specialty Business Segment [Member]
Revenue from External Customer [Line Items]
Insurance Services Revenue	306	305	914	913
Other Products [Member] | Commercial Specialty Business Segment [Member]
Revenue from External Customer [Line Items]
Insurance Services Revenue	$ 111	$ 73	$ 313	$ 272