Segment Information - Reconciliation to Adjusted Income From Operations (Details) - USD ($) $ in Millions	3 Months Ended															12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Segment Reporting Information [Line Items]
Shareholders' Net Income	$ 382	[1],[2]	$ 456	[1],[3]	$ 510	[1]	$ 519	[1]	$ 426	[1]	$ 547	[1]	$ 588	[4]	$ 533	$ 1,867	$ 2,094		$ 2,102
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses																(109)	(40)		(106)
Amortization of other acquired intangible assets																94	80	[5]	119
Special items:
Charges associated with litigation matters			25													25
Debt extinguishment costs													$ 65				65
Merger-related transaction costs	39		$ 46		$ 26		$ 36		$ 28		$ 29					147	57
Risk corridor allowance	$ 80															80
Adjusted income from operations																2,104	2,256		2,115
Operating Segments [Member] | Global Health Benefits Segment [Member]
Segment Reporting Information [Line Items]
Shareholders' Net Income																1,751	1,794		1,700
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses																(78)	(30)		(54)
Amortization of other acquired intangible assets																74	84		106
Special items:
Charges associated with litigation matters																25
Debt extinguishment costs																	0
Merger-related transaction costs																0	0
Risk corridor allowance																80
Adjusted income from operations																1,852	1,848		1,752
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Segment Reporting Information [Line Items]
Shareholders' Net Income																268	267		233
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses																6	(1)		(3)
Amortization of other acquired intangible assets																20	(4)	[5]	13
Special items:
Charges associated with litigation matters																0
Debt extinguishment costs																	0
Merger-related transaction costs																0	0
Risk corridor allowance																0
Adjusted income from operations																294	262		243
Benefit of a 2015 acquisition in which the fair value of acquired net assets exceeded the purchase price																	23
Operating Segments [Member] | Group Disability And Life Segment [Member]
Segment Reporting Information [Line Items]
Shareholders' Net Income																164	328		331
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses																(39)	(4)		(14)
Amortization of other acquired intangible assets																0	0		0
Special items:
Charges associated with litigation matters																0
Debt extinguishment costs																	0
Merger-related transaction costs																0	0
Risk corridor allowance																0
Adjusted income from operations																125	324		317
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Shareholders' Net Income																68	80		79
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses																2	(5)		(11)
Amortization of other acquired intangible assets																0	0		0
Special items:
Charges associated with litigation matters																0
Debt extinguishment costs																	0
Merger-related transaction costs																0	0
Risk corridor allowance																0
Adjusted income from operations																70	75		68
Corporate [Member]
Segment Reporting Information [Line Items]
Shareholders' Net Income																(384)	(375)		(241)
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses																0	0		(24)
Amortization of other acquired intangible assets																0	0		0
Special items:
Charges associated with litigation matters																0
Debt extinguishment costs																	65
Merger-related transaction costs																147	57
Risk corridor allowance																0
Adjusted income from operations																$ (237)	$ (253)		$ (265)

[1] Shareholders’ net income includes after-tax charges related to the Company’s proposed merger with Anthem as follows: $29 million in the third quarter of 2015, $28 million in the fourth quarter of 2015, $36 million in the first quarter of 2016, $26 million in the second quarter of 2016, $46 million in the third quarter of 2016 and $39 million in the fourth quarter of 2016. See Note 3 to the Consolidated Financial Statements for additional details.

[2] Shareholders’ net income includes an after-tax charge of $80 million for an allowance for the risk corridor receivable balance in the fourth quarter of 2016. See Note 22 to the Consolidated Financial Statements for additional details.

[3] Shareholders’ net income includes an after-tax charge of $25 million for charges related to litigation matters in the third quarter of 2016. See Note 21 to the Consolidated Financial Statements for a description of litigation matters.

[4] Shareholders’ net income includes an after-tax charge of $65 million for the early extinguishment of debt in the second quarter of 2015. See Note 5 to the Consolidated Financial Statements for additional details.

[5] Includes a $23 million bargain purchase gain for a 2015 acquisition.