Fair Value Measurements - Fair Value of Financial Instruments on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	$ 2,655	$ 3,215
Total liability measured at fair value on a recurring basis	843	522
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		101
U.S. treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		68
Subtotal - current restricted investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		169
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	1,191	1,588
U.S. treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	221	388
Government-sponsored enterprise securities (GSEs)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	170	253
Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	119	141
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	92	117
Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	15	37
Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	4
Subtotal - current investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	1,812	2,524
1.125% Call Option derivative asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	843	522
1.125% Conversion Option derivative liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
1.125% Conversion Option derivative liability	$ 843	522
1.125% Call Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of contractual interest rate on Call Option	1.125%
1.125% Conversion Option derivative liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of contractual interest rate on Call Option	1.125%
Quoted Market Prices (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	$ 391	709
Total liability measured at fair value on a recurring basis	0	0
Quoted Market Prices (Level 1) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		0
Quoted Market Prices (Level 1) | U.S. treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		68
Quoted Market Prices (Level 1) | Subtotal - current restricted investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		68
Quoted Market Prices (Level 1) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Quoted Market Prices (Level 1) | U.S. treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	221	388
Quoted Market Prices (Level 1) | Government-sponsored enterprise securities (GSEs)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	170	253
Quoted Market Prices (Level 1) | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Quoted Market Prices (Level 1) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Quoted Market Prices (Level 1) | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Quoted Market Prices (Level 1) | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0
Quoted Market Prices (Level 1) | Subtotal - current investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	391	641
Quoted Market Prices (Level 1) | 1.125% Call Option derivative asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Quoted Market Prices (Level 1) | 1.125% Conversion Option derivative liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
1.125% Conversion Option derivative liability	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	1,421	1,984
Total liability measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		101
Significant Other Observable Inputs (Level 2) | U.S. treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		0
Significant Other Observable Inputs (Level 2) | Subtotal - current restricted investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		101
Significant Other Observable Inputs (Level 2) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	1,191	1,588
Significant Other Observable Inputs (Level 2) | U.S. treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2) | Government-sponsored enterprise securities (GSEs)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2) | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	119	141
Significant Other Observable Inputs (Level 2) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	92	117
Significant Other Observable Inputs (Level 2) | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	15	37
Significant Other Observable Inputs (Level 2) | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	4
Significant Other Observable Inputs (Level 2) | Subtotal - current investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	1,421	1,883
Significant Other Observable Inputs (Level 2) | 1.125% Call Option derivative asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2) | 1.125% Conversion Option derivative liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
1.125% Conversion Option derivative liability	0	0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	843	522
Total liability measured at fair value on a recurring basis	843	522
Significant Unobservable Inputs (Level 3) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		0
Significant Unobservable Inputs (Level 3) | U.S. treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		0
Significant Unobservable Inputs (Level 3) | Subtotal - current restricted investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis		0
Significant Unobservable Inputs (Level 3) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | U.S. treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Government-sponsored enterprise securities (GSEs)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0
Significant Unobservable Inputs (Level 3) | Subtotal - current investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | 1.125% Call Option derivative asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	843	522
Significant Unobservable Inputs (Level 3) | 1.125% Conversion Option derivative liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
1.125% Conversion Option derivative liability	$ 843	$ 522