Supplemental Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 2,842	$ 2,329	$ 2,164	$ 1,539
Investments	1,735	1,801
Receivables	1,053	597
Income taxes refundable	0	13
Due from (to) affiliates	0	0
Prepaid expenses and other current assets	169	192
Derivative asset	314	374
Total current assets	6,113	5,306
Property, equipment, and capitalized software, net	450	393
Deferred contract costs	83	81
Intangible assets, net	149	122
Goodwill	619	519
Goodwill and intangible assets, net		641
Restricted investments	116	109
Investment in subsidiaries, net	0	0
Deferred income taxes	0	18
Other assets	40	28
Total Assets	7,570	6,576
Current liabilities:
Medical claims and benefits payable	1,871	1,685		1,201
Amounts due government agencies	1,232	729
Accounts payable and accrued liabilities	383	362
Deferred revenue	380	223
Income taxes payable	19	0
Current portion of long-term debt	466	449
Derivative liability	314	374
Total current liabilities	4,665	3,822
Long-term debt	1,169	1,160
Deferred income taxes	6	0
Other long-term liabilities	39	37
Total liabilities	5,879	5,019
Total stockholders' equity	1,691	1,557
Total liabilities and stockholders' equity	7,570	6,576
Eliminations
Current assets:
Cash and cash equivalents	0	0	0	0
Investments	0	0
Receivables	0	0
Income taxes refundable		0
Due from (to) affiliates	0	0
Prepaid expenses and other current assets	(1)	(1)
Derivative asset	0	0
Total current assets	(1)	(1)
Property, equipment, and capitalized software, net	0	0
Deferred contract costs	0	0
Intangible assets, net	0
Goodwill	0
Goodwill and intangible assets, net		0
Restricted investments	0	0
Investment in subsidiaries, net	(2,527)	(2,206)
Deferred income taxes		0
Other assets	(16)	(16)
Total Assets	(2,544)	(2,223)
Current liabilities:
Medical claims and benefits payable	0	0
Amounts due government agencies	0	0
Accounts payable and accrued liabilities	(1)	0
Deferred revenue	0	0
Income taxes payable	0
Current portion of long-term debt	0	0
Derivative liability	0	0
Total current liabilities	(1)	0
Long-term debt	(16)	(16)
Deferred income taxes	0
Other long-term liabilities	0	(1)
Total liabilities	(17)	(17)
Total stockholders' equity	(2,527)	(2,206)
Total liabilities and stockholders' equity	(2,544)	(2,223)
Parent Guarantor | Reportable Legal Entities
Current assets:
Cash and cash equivalents	125	360	410	75
Investments	263	252
Receivables	3	0
Income taxes refundable		7
Due from (to) affiliates	98	86
Prepaid expenses and other current assets	51	46
Derivative asset	314	374
Total current assets	854	1,125
Property, equipment, and capitalized software, net	300	267
Deferred contract costs	0	0
Intangible assets, net	7
Goodwill	51
Goodwill and intangible assets, net		61
Restricted investments	0	0
Investment in subsidiaries, net	2,526	2,205
Deferred income taxes		23
Other assets	47	36
Total Assets	3,785	3,717
Current liabilities:
Medical claims and benefits payable	0	0
Amounts due government agencies	0	0
Accounts payable and accrued liabilities	146	157
Deferred revenue	0	0
Income taxes payable	(13)
Current portion of long-term debt	466	449
Derivative liability	314	374
Total current liabilities	913	980
Long-term debt	1,169	1,160
Deferred income taxes	(7)
Other long-term liabilities	19	20
Total liabilities	2,094	2,160
Total stockholders' equity	1,691	1,557
Total liabilities and stockholders' equity	3,785	3,717
Other Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	37	42	32	15
Investments	0	0
Receivables	84	79
Income taxes refundable		3
Due from (to) affiliates	(5)	(4)
Prepaid expenses and other current assets	37	11
Derivative asset	0	0
Total current assets	153	131
Property, equipment, and capitalized software, net	69	52
Deferred contract costs	83	81
Intangible assets, net	21
Goodwill	231
Goodwill and intangible assets, net		246
Restricted investments	0	0
Investment in subsidiaries, net	1	1
Deferred income taxes		(35)
Other assets	4	2
Total Assets	562	478
Current liabilities:
Medical claims and benefits payable	0	3
Amounts due government agencies	0	1
Accounts payable and accrued liabilities	53	35
Deferred revenue	42	34
Income taxes payable	(5)
Current portion of long-term debt	0	0
Derivative liability	0	0
Total current liabilities	90	73
Long-term debt	0	0
Deferred income taxes	41
Other long-term liabilities	2	2
Total liabilities	133	75
Total stockholders' equity	429	403
Total liabilities and stockholders' equity	562	478
Non-Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	2,680	1,927	$ 1,722	$ 1,449
Investments	1,472	1,549
Receivables	966	518
Income taxes refundable		3
Due from (to) affiliates	(93)	(82)
Prepaid expenses and other current assets	82	136
Derivative asset	0	0
Total current assets	5,107	4,051
Property, equipment, and capitalized software, net	81	74
Deferred contract costs	0	0
Intangible assets, net	121
Goodwill	337
Goodwill and intangible assets, net		334
Restricted investments	116	109
Investment in subsidiaries, net	0	0
Deferred income taxes		30
Other assets	5	6
Total Assets	5,767	4,604
Current liabilities:
Medical claims and benefits payable	1,871	1,682
Amounts due government agencies	1,232	728
Accounts payable and accrued liabilities	185	170
Deferred revenue	338	189
Income taxes payable	37
Current portion of long-term debt	0	0
Derivative liability	0	0
Total current liabilities	3,663	2,769
Long-term debt	16	16
Deferred income taxes	(28)
Other long-term liabilities	18	16
Total liabilities	3,669	2,801
Total stockholders' equity	2,098	1,803
Total liabilities and stockholders' equity	$ 5,767	$ 4,604