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Fair Value Measurements - Fair Value of Financial Instruments on Recurring Basis (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value on a recurring basis $ 2,024 $ 2,157
Total liability measured at fair value on a recurring basis 516 476
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 947 1,123
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 168 181
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 166 163
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 112 114
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 75 82
Mortgage-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 23  
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 14 14
Other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value on a recurring basis 3 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 1,508 1,681
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 516 476
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 516 476
Observable Inputs (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 0 0
Total liability measured at fair value on a recurring basis 0 0
Observable Inputs (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
Observable Inputs (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 0 0
Observable Inputs (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 0 0
Observable Inputs (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
Observable Inputs (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
Observable Inputs (Level 1) | Mortgage-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  
Observable Inputs (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
Observable Inputs (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 0
Observable Inputs (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 0 0
Observable Inputs (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
Observable Inputs (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
Directly or Indirectly 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 1,508 1,681
Total liability measured at fair value on a recurring basis 0 0
Directly or Indirectly 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 947 1,123
Directly or Indirectly 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 168 181
Directly or Indirectly 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 166 163
Directly or Indirectly 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 112 114
Directly or Indirectly 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 75 82
Directly or Indirectly Observable Inputs (Level 2) | Mortgage-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 23  
Directly or Indirectly 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 14 14
Directly or Indirectly 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 3 4
Directly or Indirectly 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 1,508 1,681
Directly or Indirectly 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
Directly or Indirectly 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
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 516 476
Total liability measured at fair value on a recurring basis 516 476
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
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
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
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
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
Unobservable Inputs (Level 3) | Mortgage-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  
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
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 0
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
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 516 476
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 $ 516 $ 476