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Investment Securities (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
AvailableForSaleSecuritiesAbstract |  |
Cost | $ 6,448 | $ 4,659
Gross Unrealized Gains | 22 | 10
Gross Unrealized Losses | (13) | (22)
Estimated Fair Value | 6,457 | 4,647
U.S. States and Political Subdivisions Debt Securities [Member] |  |
AvailableForSaleSecuritiesAbstract |  |
Cost | 342 | 594
Gross Unrealized Gains | 6 | 4
Gross Unrealized Losses | (1) | (2)
Estimated Fair Value | 347 | 596
U.S. Government agency obligations [Member] |  |
AvailableForSaleSecuritiesAbstract |  |
Cost | 10 | 10
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 10 | 10
U.S. Government treasury obligations [Member] |  |
AvailableForSaleSecuritiesAbstract |  |
Cost | 5,404 | 3,452
Gross Unrealized Gains | 14 | 5
Gross Unrealized Losses | (9) | (17)
Estimated Fair Value | 5,409 | 3,440
Corporate debt securities [Member] |  |
AvailableForSaleSecuritiesAbstract |  |
Cost | 29 | 28
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 29 | 28
Mortgage-backed securities [Member] |  |
AvailableForSaleSecuritiesAbstract |  |
Cost | 48 | 50
Gross Unrealized Gains | 1 | 1
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 49 | 51
Equity securities [Member] |  |
AvailableForSaleSecuritiesAbstract |  |
Cost | 50 | 51
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (3) | (3)
Estimated Fair Value | 47 | 48
Foreign government bonds and obligations [Member] |  |
AvailableForSaleSecuritiesAbstract |  |
Cost | 565 | 474
Gross Unrealized Gains | 1 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | $ 566 | $ 474

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