Segment Information - Summarized Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	$ 10,036	$ 9,644
Net investment income	303	272
Total operating revenues	10,339	9,916
Total revenues	10,385	9,884
Shareholders' Net Income	598	519
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(31)	21
Amortization of other acquired intangible assets	20	25
Special items:
Long-term care guaranty fund assessment	83	0
Merger-related transaction costs	49	36
Adjusted income from operations	719	601
Operating Segments [Member] | Global Health Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	8,106	7,812
Net investment income	92	72
Total operating revenues	8,198	7,884
Total revenues	8,224	7,867
Shareholders' Net Income	544	514
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(16)	12
Amortization of other acquired intangible assets	14	18
Special items:
Long-term care guaranty fund assessment	68
Merger-related transaction costs	0	0
Adjusted income from operations	610	544
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	881	780
Net investment income	28	26
Total operating revenues	909	806
Total revenues	922	804
Shareholders' Net Income	77	59
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(9)	1
Amortization of other acquired intangible assets	6	7
Special items:
Long-term care guaranty fund assessment	0
Merger-related transaction costs	0	0
Adjusted income from operations	74	67
Operating Segments [Member] | Group Disability And Life Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,032	1,029
Net investment income	89	80
Total operating revenues	1,121	1,109
Total revenues	1,129	1,106
Shareholders' Net Income	59	13
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(6)	2
Amortization of other acquired intangible assets	0	0
Special items:
Long-term care guaranty fund assessment	15
Merger-related transaction costs	0	0
Adjusted income from operations	68	15
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	30	27
Net investment income	86	90
Total operating revenues	116	117
Total revenues	115	107
Shareholders' Net Income	20	14
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	0	6
Amortization of other acquired intangible assets	0	0
Special items:
Long-term care guaranty fund assessment	0
Merger-related transaction costs	0	0
Adjusted income from operations	20	20
Corporate [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	(13)	(4)
Net investment income	8	4
Total operating revenues	(5)	0
Total revenues	(5)	0
Shareholders' Net Income	(102)	(81)
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	0	0
Amortization of other acquired intangible assets	0	0
Special items:
Long-term care guaranty fund assessment	0
Merger-related transaction costs	49	36
Adjusted income from operations	$ (53)	$ (45)