Litigation (Details Textual) $ in Millions	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 Homebuilder
Commitments and Contingencies Disclosure [Abstract]
Number of Defendants Named in Class Action Lawsuits			7
Homebuilders Asserting Individual Claims			12
Accrual for probable and reasonably estimable liability for environmental cleanup | $	$ 17	$ 18