Supplemental Condensed Consolidating Financial Information - Balance Sheet (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 3,392	$ 3,186	$ 2,979
Investments	2,176	2,524
Restricted investments	80	169
Receivables	1,148	871
Due from (to) affiliates	0	0
Prepaid expenses and other current assets	344	239
Derivative asset	657	522
Assets held for sale	230	0
Total current assets	8,027	7,511
Property, equipment, and capitalized software, net	276	342
Goodwill and intangible assets, net	201	255
Restricted investments	117	119
Investment in subsidiaries, net	0	0
Deferred income taxes	114	103
Other assets	28	141
Total assets held for sale	8,763	8,471
Current liabilities:
Medical claims and benefits payable	1,920	2,192	$ 2,077	$ 1,929
Amounts due government agencies	1,746	1,542
Accounts payable and accrued liabilities	754	366
Deferred revenue	193	282
Current portion of long-term debt	484	653
Derivative liability	657	522
Liabilities held for sale	66	0
Total current liabilities	5,820	5,557
Long-term debt	1,217	1,516
Deferred income taxes	0	0
Other long-term liabilities	68	61
Total liabilities	7,105	7,134
Total stockholders’ equity	1,658	1,337
Total liabilities and stockholders' equity	8,763	8,471
Eliminations
Current assets:
Cash and cash equivalents	0	0
Investments	0	0
Restricted investments	0	0
Receivables	0	0
Due from (to) affiliates	0	0
Prepaid expenses and other current assets	0	(16)
Derivative asset	0	0
Assets held for sale	0
Total current assets	0	(16)
Property, equipment, and capitalized software, net	0	0
Goodwill and intangible assets, net	0	0
Restricted investments	0	0
Investment in subsidiaries, net	(2,837)	(2,388)
Deferred income taxes	(18)	(15)
Other assets	(16)	(1)
Total assets held for sale	(2,871)	(2,420)
Current liabilities:
Medical claims and benefits payable	0	0
Amounts due government agencies	0	0
Accounts payable and accrued liabilities	0	0
Deferred revenue	0	0
Current portion of long-term debt	0	(16)
Derivative liability	0	0
Liabilities held for sale	0
Total current liabilities	0	(16)
Long-term debt	(16)	0
Deferred income taxes	(18)	(15)
Other long-term liabilities	0	(1)
Total liabilities	(34)	(32)
Total stockholders’ equity	(2,837)	(2,388)
Total liabilities and stockholders' equity	(2,871)	(2,420)
Parent Guarantor | Reportable Legal Entities
Current assets:
Cash and cash equivalents	235	504
Investments	105	192
Restricted investments	80	169
Receivables	2	2
Due from (to) affiliates	68	148
Prepaid expenses and other current assets	62	103
Derivative asset	657	522
Assets held for sale	0
Total current assets	1,209	1,640
Property, equipment, and capitalized software, net	196	223
Goodwill and intangible assets, net	14	15
Restricted investments	0	0
Investment in subsidiaries, net	2,761	2,306
Deferred income taxes	33	17
Other assets	39	32
Total assets held for sale	4,252	4,233
Current liabilities:
Medical claims and benefits payable	2	3
Amounts due government agencies	0	0
Accounts payable and accrued liabilities	211	178
Deferred revenue	0	0
Current portion of long-term debt	484	653
Derivative liability	657	522
Liabilities held for sale	0
Total current liabilities	1,354	1,356
Long-term debt	1,217	1,516
Deferred income taxes	0	0
Other long-term liabilities	23	24
Total liabilities	2,594	2,896
Total stockholders’ equity	1,658	1,337
Total liabilities and stockholders' equity	4,252	4,233
Other Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	2	28
Investments	0	0
Restricted investments	0	0
Receivables	0	30
Due from (to) affiliates	(6)	(6)
Prepaid expenses and other current assets	0	14
Derivative asset	0	0
Assets held for sale	230
Total current assets	226	66
Property, equipment, and capitalized software, net	0	33
Goodwill and intangible assets, net	0	43
Restricted investments	0	0
Investment in subsidiaries, net	76	82
Deferred income taxes	0	0
Other assets	0	103
Total assets held for sale	302	327
Current liabilities:
Medical claims and benefits payable	0	0
Amounts due government agencies	0	1
Accounts payable and accrued liabilities	0	40
Deferred revenue	0	49
Current portion of long-term debt	0	0
Derivative liability	0	0
Liabilities held for sale	66
Total current liabilities	66	90
Long-term debt	0	0
Deferred income taxes	18	15
Other long-term liabilities	1	2
Total liabilities	85	107
Total stockholders’ equity	217	220
Total liabilities and stockholders' equity	302	327
Non-Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	3,155	2,654
Investments	2,071	2,332
Restricted investments	0	0
Receivables	1,146	839
Due from (to) affiliates	(62)	(142)
Prepaid expenses and other current assets	282	138
Derivative asset	0	0
Assets held for sale	0
Total current assets	6,592	5,821
Property, equipment, and capitalized software, net	80	86
Goodwill and intangible assets, net	187	197
Restricted investments	117	119
Investment in subsidiaries, net	0	0
Deferred income taxes	99	101
Other assets	5	7
Total assets held for sale	7,080	6,331
Current liabilities:
Medical claims and benefits payable	1,918	2,189
Amounts due government agencies	1,746	1,541
Accounts payable and accrued liabilities	543	148
Deferred revenue	193	233
Current portion of long-term debt	0	16
Derivative liability	0	0
Liabilities held for sale	0
Total current liabilities	4,400	4,127
Long-term debt	16	0
Deferred income taxes	0	0
Other long-term liabilities	44	36
Total liabilities	4,460	4,163
Total stockholders’ equity	2,620	2,168
Total liabilities and stockholders' equity	$ 7,080	$ 6,331