Segment Information (Major Product Revenues) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Reportable segments operating revenues	$ 89,061.2	$ 84,194.0	$ 78,404.8
Commercial and Specialty Business Segment [Member]
Revenue from External Customer [Line Items]
Reportable segments operating revenues	40,754.1	38,692.1	37,570.8
Commercial and Specialty Business Segment [Member] | Managed Care Products [Member]
Revenue from External Customer [Line Items]
Reportable segments operating revenues	34,272.1	32,369.8	31,676.9
Commercial and Specialty Business Segment [Member] | Managed Care Services [Member]
Revenue from External Customer [Line Items]
Reportable segments operating revenues	4,821.0	4,710.1	4,344.8
Commercial and Specialty Business Segment [Member] | Dental/Vision Products And Services [Member]
Revenue from External Customer [Line Items]
Reportable segments operating revenues	1,218.1	1,182.3	1,111.7
Commercial and Specialty Business Segment [Member] | Other Products and Services [Member]
Revenue from External Customer [Line Items]
Reportable segments operating revenues	442.9	429.9	437.4
Government Business Segment [Member]
Revenue from External Customer [Line Items]
Reportable segments operating revenues	48,276.2	45,477.7	40,813.0
Government Business Segment [Member] | Managed Care Products [Member]
Revenue from External Customer [Line Items]
Reportable segments operating revenues	47,843.4	45,028.5	40,307.0
Government Business Segment [Member] | Managed Care Services [Member]
Revenue from External Customer [Line Items]
Reportable segments operating revenues	432.8	449.2	506.0
Other [Member]
Revenue from External Customer [Line Items]
Reportable segments operating revenues	$ 30.9	$ 24.2	$ 21.0