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Other Financial Data - Components of Fixed Assets, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fixed Assets, net
Fixed assets, gross	$ 1,733	$ 1,733
Less: Accumulated depreciation	1,145	1,169
Fixed assets, net	588	564
Depreciation of fixed assets	176	187	$ 162
Software
Fixed Assets, net
Fixed assets, gross	693	680
Leasehold improvements
Fixed Assets, net
Fixed assets, gross	334	349
Computer equipment
Fixed Assets, net
Fixed assets, gross	279	295
Furniture, fixtures and equipment
Fixed Assets, net
Fixed assets, gross	228	240
Construction in progress
Fixed Assets, net
Fixed assets, gross	154	79
Other
Fixed Assets, net
Fixed assets, gross	$ 45	$ 90