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Other Assets - Additional information (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entity, Not Primary Beneficiary
Schedule of Equity Method Investments [Line Items]
Strategic investment	$ 3		$ 3
Maximum exposure to loss	9	$ 6
Equity Method Joint Venture
Schedule of Equity Method Investments [Line Items]
Investments in joint venture	$ 68	$ 63