Fair Value Measurements - Financial Instruments on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	$ 1,946	$ 1,681
Total assets	1,975	2,157
Total liabilities	29	476
Corporate debt securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	1,178	1,123
Mortgage-backed securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	420
Asset-backed securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	127	82
U.S. Treasury notes
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	86	181
Municipal securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	78	114
Government-sponsored enterprise securities (“GSEs”)
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	49	163
Foreign securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	7	4
Certificates of deposit
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	1	14
1.125% Call Option derivative asset
Fair value of assets measured on recurring basis
1.125% Call Option derivative asset	$ 29	476
Percentage of contractual interest rate on derivative	1.125%
Conversion Option
Fair value of assets measured on recurring basis
1.125% Conversion Option derivative liability	$ 29	476
Percentage of contractual interest rate on derivative	1.125%
Level 1
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	$ 0	0
Total assets	0	0
Total liabilities	0	0
Level 1 \| Corporate debt securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 1 \| Mortgage-backed securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0
Level 1 \| Asset-backed securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 1 \| U.S. Treasury notes
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 1 \| Municipal securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 1 \| Government-sponsored enterprise securities (“GSEs”)
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 1 \| Foreign securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 1 \| Certificates of deposit
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 1 \| 1.125% Call Option derivative asset
Fair value of assets measured on recurring basis
1.125% Call Option derivative asset	0	0
Level 1 \| 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
Debt securities, available-for-sale	1,946	1,681
Total assets	1,946	1,681
Total liabilities	0	0
Level 2 \| Corporate debt securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	1,178	1,123
Level 2 \| Mortgage-backed securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	420
Level 2 \| Asset-backed securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	127	82
Level 2 \| U.S. Treasury notes
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	86	181
Level 2 \| Municipal securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	78	114
Level 2 \| Government-sponsored enterprise securities (“GSEs”)
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	49	163
Level 2 \| Foreign securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	7	4
Level 2 \| Certificates of deposit
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	1	14
Level 2 \| 1.125% Call Option derivative asset
Fair value of assets measured on recurring basis
1.125% Call Option derivative asset	0	0
Level 2 \| 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
Debt securities, available-for-sale	0	0
Total assets	29	476
Total liabilities	29	476
Level 3 \| Corporate debt securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 3 \| Mortgage-backed securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0
Level 3 \| Asset-backed securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 3 \| U.S. Treasury notes
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 3 \| Municipal securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 3 \| Government-sponsored enterprise securities (“GSEs”)
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 3 \| Foreign securities
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 3 \| Certificates of deposit
Fair value of assets measured on recurring basis
Debt securities, available-for-sale	0	0
Level 3 \| 1.125% Call Option derivative asset
Fair value of assets measured on recurring basis
1.125% Call Option derivative asset	29	476
Level 3 \| Conversion Option
Fair value of assets measured on recurring basis
1.125% Conversion Option derivative liability	$ 29	$ 476

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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- Definition

Fixed interest rate related to the interest rate derivative.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of financial and nonfinancial obligations.

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