Fair Value Measurements - Fair Value of Financial Instruments on Recurring Basis (Details) - USD ($) $ in Millions | Mar. 31, 2020 | Dec. 31, 2019 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | $ 2,010 | $ 1,946 |
Total assets | 2,010 | 1,975 |
Total liabilities | | 29 |
Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 1,260 | 1,178 |
Mortgage-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 451 | 420 |
Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 143 | 127 |
Municipal securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 138 | 78 |
Certificates of deposit | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 7 | 1 |
Government-sponsored enterprise securities (“GSEs”) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 6 | 49 |
U.S. Treasury notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 5 | 86 |
Foreign securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | | 7 |
Call option derivative asset | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Call option derivative asset | | 29 |
Conversion option derivative liability | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Conversion option derivative liability | | 29 |
Observable Inputs (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | | 0 |
Total assets | 0 | 0 |
Total liabilities | | 0 |
Observable Inputs (Level 1) | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Observable Inputs (Level 1) | Mortgage-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Observable Inputs (Level 1) | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Observable Inputs (Level 1) | Municipal securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Observable Inputs (Level 1) | Certificates of deposit | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Observable Inputs (Level 1) | Government-sponsored enterprise securities (“GSEs”) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Observable Inputs (Level 1) | U.S. Treasury notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Observable Inputs (Level 1) | Foreign securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | | 0 |
Observable Inputs (Level 1) | Call option derivative asset | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Call option derivative asset | | 0 |
Observable Inputs (Level 1) | Conversion option derivative liability | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Conversion option derivative liability | | 0 |
Directly or Indirectly Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | | 1,946 |
Total assets | 2,010 | 1,946 |
Total liabilities | | 0 |
Directly or Indirectly Observable Inputs (Level 2) | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 1,260 | 1,178 |
Directly or Indirectly Observable Inputs (Level 2) | Mortgage-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 451 | 420 |
Directly or Indirectly Observable Inputs (Level 2) | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 143 | 127 |
Directly or Indirectly Observable Inputs (Level 2) | Municipal securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 138 | 78 |
Directly or Indirectly Observable Inputs (Level 2) | Certificates of deposit | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 7 | 1 |
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] | | |
Debt securities, available-for-sale | 6 | 49 |
Directly or Indirectly Observable Inputs (Level 2) | U.S. Treasury notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 5 | 86 |
Directly or Indirectly Observable Inputs (Level 2) | Foreign securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | | 7 |
Directly or Indirectly Observable Inputs (Level 2) | Call option derivative asset | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Call option derivative asset | | 0 |
Directly or Indirectly Observable Inputs (Level 2) | Conversion option derivative liability | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Conversion option derivative liability | | 0 |
Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | | 0 |
Total assets | 0 | 29 |
Total liabilities | | 29 |
Unobservable Inputs (Level 3) | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Unobservable Inputs (Level 3) | Mortgage-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Unobservable Inputs (Level 3) | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Unobservable Inputs (Level 3) | Municipal securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Unobservable Inputs (Level 3) | Certificates of deposit | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Unobservable Inputs (Level 3) | Government-sponsored enterprise securities (“GSEs”) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | 0 | 0 |
Unobservable Inputs (Level 3) | U.S. Treasury notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | $ 0 | 0 |
Unobservable Inputs (Level 3) | Foreign securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, available-for-sale | | 0 |
Unobservable Inputs (Level 3) | Call option derivative asset | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Call option derivative asset | | 29 |
Unobservable Inputs (Level 3) | Conversion option derivative liability | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Conversion option derivative liability | | $ 29 |