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Investments - Summary of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,512
Gross Unrealized Gains	1
Gross Unrealized Losses	5
Estimated Fair Value	1,508
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	949	$ 1,131
Gross Unrealized Gains	1	0
Gross Unrealized Losses	3	8
Estimated Fair Value	947	1,123
U.S. Treasury notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	168	181
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	168	181
GSEs
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	167	164
Gross Unrealized Gains	0	0
Gross Unrealized Losses	1	1
Estimated Fair Value	166	163
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	113	115
Gross Unrealized Gains	0	0
Gross Unrealized Losses	1	1
Estimated Fair Value	112	114
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	75	83
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	1
Estimated Fair Value	75	82
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	23
Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14	14
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	14	14
Other
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3	4
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 3	4
Subtotal - current investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,692
Gross Unrealized Gains		0
Gross Unrealized Losses		11
Estimated Fair Value		$ 1,681

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