Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Accrual for estimated environmental remediation costs	$ 6,590	$ 8,490	$ 9,441	$ 12,770	$ 16,761	$ 21,651
Environmental liability, next 12 months	3,400
Environmental liability thereafter	$ 3,200