Litigation (Details) $ in Millions	Dec. 31, 2017 USD ($) Homebuilder Defendants	Dec. 31, 2016 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Number of defendants named in lawsuit | Defendants	7
Number of homebuilders asserting individual claims | Homebuilder	12
Accrual potential liability for environmental cleanup | $	$ 17	$ 18