Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities:
Net income (loss)	$ 737	$ 707	$ (512)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	89	127	178
Deferred income taxes	10	(6)	(94)
Share-based compensation	39	27	46
Amortization of convertible senior notes and finance lease liabilities	5	22	32
(Gain) loss on debt extinguishment	(15)	22	14
Loss on sales of subsidiaries, net of gain	0	15	0
Non-cash restructuring charges	0	17	60
Impairment losses	0	0	470
Other, net	(5)	4	21
Changes in operating assets and liabilities:
Receivables	(76)	(530)	103
Prepaid expenses and other current assets	28	6	(56)
Medical claims and benefits payable	(107)	(226)	263
Amounts due government agencies	(303)	(574)	341
Accounts payable and accrued liabilities	2	45	(12)
Deferred revenue	38	(21)	(34)
Income taxes	(15)	51	(16)
Net cash provided by (used in) operating activities	427	(314)	804
Investing activities:
Purchases of investments	(2,536)	(1,444)	(2,697)
Proceeds from sales and maturities of investments	2,302	2,445	1,759
Purchases of property, equipment and capitalized software	(57)	(30)	(86)
Net cash received from sale of subsidiaries	0	190	0
Other, net	(2)	(18)	(38)
Net cash provided by (used in) investing activities	(293)	1,143	(1,062)
Financing activities:
Repayment of principal amount of convertible senior notes	(240)	(362)	0
Cash paid for partial settlement of conversion option	(578)	(623)	0
Cash received for partial settlement of call option	578	623	0
Cash paid for partial termination of warrants	(514)	(549)	0
Proceeds from borrowings under term loan facility	220	0	0
Common stock purchases	(47)	0	0
Repayment of credit facility	0	(300)	0
Proceeds from senior notes offerings, net of issuance costs	0	0	325
Proceeds from borrowings under credit facility	0	0	300
Other, net	29	18	11
Net cash (used in) provided by financing activities	(552)	(1,193)	636
Net (decrease) increase in cash and cash equivalents, and restricted cash and cash equivalents	(418)	(364)	378
Cash and cash equivalents, and restricted cash and cash equivalents at beginning of period	2,926	3,290	2,912
Cash and cash equivalents at end of period	2,508	2,926	3,290
Cash paid during the period for:
Income taxes	239	240	7
Interest	78	93	78
Schedule of non-cash investing and financing activities:
Common stock issued in exchange for convertible senior notes	0	131	193
Component of convertible senior notes allocated to additional paid-in capital, net of income taxes	0	(23)	(32)
Net increase to additional paid-in capital	0	108	161
Common stock used for stock-based compensation	(7)	(6)	(22)
Common stock purchases not settled at end of period	7	0	0
Details of sales of subsidiaries:
Decrease in carrying amount of assets	0	(327)	0
Decrease in carrying amount of liabilities	0	85	0
Transaction costs	0	(15)	0
Cash received from buyers	0	242	0
Loss on sale of subsidiaries, net of gain	0	(15)	0
Details of change in fair value of derivatives, net:
Change in fair value of derivatives, net	0	0	0
Call Option
Details of change in fair value of derivatives, net:
Change in fair value of derivatives, net	132	577	255
Conversion Option
Details of change in fair value of derivatives, net:
Change in fair value of derivatives, net	$ (132)	$ (577)	$ (255)