Investments - Contractual Maturities of Investments (Details) $ in Millions	Mar. 31, 2019 USD ($)
Amortized Cost
Due in one year or less	$ 927
Due after one year through five years	550
Due after five years through ten years	12
Due after ten years	23
Amortized Cost	1,512
Estimated Fair Value
Due in one year or less	926
Due after one year through five years	547
Due after five years through ten years	12
Due after ten years	23
Estimated Fair Value	$ 1,508

Amortized cost 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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