Commitments and Contingencies (Details Textuals) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments And Contingencies (Textuals) [Abstract]
Amount of rentals subject to subleasing arrangements	$ 20
Future minimum payments on capital leases due, in 2018	4
Future minimum payments on capital leases due, in 2019	4
Future minimum payments on capital leases due, in 2020	4
Future minimum payments on capital leases due, in 2021	2
Future minimum payments on capital leases due, in 2022	1
Future minimum payments on capital leases due, thereafter	10
Total rental expense	151	$ 169	$ 187
Loss Contingencies [Line Items]
Loss Contingency Estimate Of Possible Loss	500
Minimum [Member]
Loss Contingencies [Line Items]
Loss Contingency Estimate Of Possible Loss	$ 0