Segment Information - Summarized Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	$ 11,084	$ 10,125
Net investment income	329	303
Net realized investment gains (losses) on equity method subsidiaries	(2)
Total operating revenues	11,415	10,428
Total revenues	11,380	10,474
Shareholders' Net Income	915	598
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	25	(31)
Amortization of other acquired intangible assets, net	20	20
Special items:
Long-term care guaranty fund assessment	0	83
Transaction-related costs	50	49
Total special items	50	132
Adjusted income (loss) from operations	1,010	719
Operating Segments [Member] | Global Health Care [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	8,974	8,195
Net investment income	115	92
Net realized investment gains (losses) on equity method subsidiaries	0
Total operating revenues	9,089	8,287
Total revenues	9,069	8,313
Shareholders' Net Income	842	544
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	15	(16)
Amortization of other acquired intangible assets, net	14	14
Special items:
Long-term care guaranty fund assessment		68
Transaction-related costs	0	0
Total special items		68
Adjusted income (loss) from operations	871	610
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,066	881
Net investment income	34	28
Net realized investment gains (losses) on equity method subsidiaries	(2)
Total operating revenues	1,102	909
Total revenues	1,100	922
Shareholders' Net Income	105	77
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	1	(9)
Amortization of other acquired intangible assets, net	6	6
Special items:
Long-term care guaranty fund assessment		0
Transaction-related costs	0	0
Total special items		0
Adjusted income (loss) from operations	112	74
Operating Segments [Member] | Group Disability And Life Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,026	1,032
Net investment income	90	89
Net realized investment gains (losses) on equity method subsidiaries	0
Total operating revenues	1,116	1,121
Total revenues	1,100	1,129
Shareholders' Net Income	54	59
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	13	(6)
Amortization of other acquired intangible assets, net	0	0
Special items:
Long-term care guaranty fund assessment		15
Transaction-related costs	0	0
Total special items		15
Adjusted income (loss) from operations	67	68
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	31	30
Net investment income	85	86
Net realized investment gains (losses) on equity method subsidiaries	0
Total operating revenues	116	116
Total revenues	120	115
Shareholders' Net Income	29	20
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	(5)	0
Amortization of other acquired intangible assets, net	0	0
Special items:
Long-term care guaranty fund assessment		0
Transaction-related costs	0	0
Total special items		0
Adjusted income (loss) from operations	24	20
Corporate [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	(13)	(13)
Net investment income	5	8
Net realized investment gains (losses) on equity method subsidiaries	0
Total operating revenues	(8)	(5)
Total revenues	(9)	(5)
Shareholders' Net Income	(115)	(102)
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	1	0
Amortization of other acquired intangible assets, net	0	0
Special items:
Long-term care guaranty fund assessment		0
Transaction-related costs	50	49
Total special items		49
Adjusted income (loss) from operations	$ (64)	$ (53)