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Fair Value Measurements - Financial Instruments on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | $ 3,215 | $ 2,025
Total liabilities measured at fair value on a recurring basis | 522 | 267
Corporate debt securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 101 |
U.S. treasury notes |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 68 |
Subtotal - current restricted investments |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 169 |
Corporate debt securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 1,588 | 1,179
U.S. treasury notes |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 388 | 84
Government-sponsored enterprise securities (GSEs) |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 253 | 231
Municipal securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 141 | 142
Asset-backed securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 117 | 69
Certificates of deposit |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 37 | 53
Subtotal - current investments |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 2,524 | 1,758
1.125% Call Option derivative asset |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 522 | 267
1.125% Conversion Option |  |
Fair value of assets measured on recurring basis |  |
1.125% Conversion Option derivative liability | 522 | 267
Level 1 |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 709 | 315
Total liabilities measured at fair value on a recurring basis | 0 | 0
Level 1 | Corporate debt securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 |
Level 1 | U.S. treasury notes |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 68 |
Level 1 | Subtotal - current restricted investments |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 68 |
Level 1 | Corporate debt securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 1 | U.S. treasury notes |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 388 | 84
Level 1 | Government-sponsored enterprise securities (GSEs) |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 253 | 231
Level 1 | Municipal securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 1 | Asset-backed securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 1 | Certificates of deposit |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 1 | Subtotal - current investments |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 641 | 315
Level 1 | 1.125% Call Option derivative asset |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 1 | 1.125% Conversion Option |  |
Fair value of assets measured on recurring basis |  |
1.125% Conversion Option derivative liability | 0 | 0
Level 2 |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 1,984 | 1,443
Total liabilities measured at fair value on a recurring basis | 0 | 0
Level 2 | Corporate debt securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 101 |
Level 2 | U.S. treasury notes |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 |
Level 2 | Subtotal - current restricted investments |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 101 |
Level 2 | Corporate debt securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 1,588 | 1,179
Level 2 | U.S. treasury notes |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 2 | Government-sponsored enterprise securities (GSEs) |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 2 | Municipal securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 141 | 142
Level 2 | Asset-backed securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 117 | 69
Level 2 | Certificates of deposit |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 37 | 53
Level 2 | Subtotal - current investments |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 1,883 | 1,443
Level 2 | 1.125% Call Option derivative asset |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 2 | 1.125% Conversion Option |  |
Fair value of assets measured on recurring basis |  |
1.125% Conversion Option derivative liability | 0 | 0
Level 3 |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 522 | 267
Total liabilities measured at fair value on a recurring basis | 522 | 267
Level 3 | Corporate debt securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 |
Level 3 | U.S. treasury notes |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 |
Level 3 | Subtotal - current restricted investments |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 |
Level 3 | Corporate debt securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 3 | U.S. treasury notes |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 3 | Government-sponsored enterprise securities (GSEs) |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 3 | Municipal securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 3 | Asset-backed securities |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 3 | Certificates of deposit |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 3 | Subtotal - current investments |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 0 | 0
Level 3 | 1.125% Call Option derivative asset |  |
Fair value of assets measured on recurring basis |  |
Total assets measured at fair value on a recurring basis | 522 | 267
Level 3 | 1.125% Conversion Option |  |
Fair value of assets measured on recurring basis |  |
1.125% Conversion Option derivative liability | $ 522 | $ 267

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