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