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Fair Value Measurements - Fair Value of Financial Instruments on Recurring Basis (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
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%  
Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value on a recurring basis $ 2,237 $ 2,025
Total liability measured at fair value on a recurring basis 181 267
Recurring | 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 1,353 1,179
Recurring | 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 209 231
Recurring | 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 193 84
Recurring | 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 152 142
Recurring | 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 110 69
Recurring | Certificates 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 39 53
Recurring | 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 2,056 1,758
Recurring | 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 181 267
Recurring | 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 181 267
Recurring | 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 402 315
Total liability measured at fair value on a recurring basis 0 0
Recurring | 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
Recurring | 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 209 231
Recurring | 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 193 84
Recurring | 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
Recurring | 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
Recurring | Level 1 | Certificates 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
Recurring | 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 402 315
Recurring | 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
Recurring | 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
Recurring | 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 1,654 1,443
Total liability measured at fair value on a recurring basis 0 0
Recurring | 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 1,353 1,179
Recurring | 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
Recurring | 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
Recurring | 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 152 142
Recurring | 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 110 69
Recurring | Level 2 | Certificates 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 39 53
Recurring | 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 1,654 1,443
Recurring | 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
Recurring | 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
Recurring | 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 181 267
Total liability measured at fair value on a recurring basis 181 267
Recurring | 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
Recurring | 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
Recurring | 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
Recurring | 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
Recurring | 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
Recurring | Level 3 | Certificates 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
Recurring | 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
Recurring | 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 181 267
Recurring | 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 $ 181 $ 267