Capital Structure (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Interest paid on debt	$ 6	$ 15	$ 16
Liabilities assumed		34
Other liabilities and accrued expenses	1,076	952
Limited Partnership
Debt Instrument [Line Items]
Liabilities assumed	$ 34	56	$ 44
Other liabilities and accrued expenses		$ 22