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Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Values of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Debt and Equity Securities
Amortized Cost	$ 194	$ 195
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	195	195
Asset-backed securities
Debt Securities
Amortized Cost	30	33
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	30	33
Corporate bonds
Debt Securities
Amortized Cost	97	99
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	98	99
U.S. government agencies and government- sponsored agencies
Debt Securities
Amortized Cost	5	7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	5	7
U.S. treasuries
Debt Securities
Amortized Cost	54	46
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	54	46
Exchange traded fund
Equity Securities
Amortized Cost	1	1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1	1
Other debt securities
Debt Securities
Amortized Cost	7	9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 7	$ 9

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