Litigation (Details) $ in Millions	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	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 | $	$ 16	$ 17