Business Acquisitions (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Apr. 30, 2018	Mar. 31, 2018	Mar. 31, 2017
Business Acquisition [Line Items]
Acquisitions, net of cash acquired		$ 0	$ (579)
Other acquisitions [Member]
Business Acquisition [Line Items]
Acquisitions, net of cash acquired		$ 579
Subsequent Event [Member] | HomeServices [Member] | Real estate brokerage franchise business [Member]
Business Acquisition [Line Items]
Ownership Percentage Acquired	33.30%
Business acquisition, acquisition of remaining noncontrolling interest	$ 131