Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies Disclosure [Abstract]

Summary of Remediation Reserve Activity

A roll forward of remediation reserves on the balance sheet is comprised of the following:

	Years Ended December 31,
	2017			2016			2015
Balance at beginning of period	$	18,176		$	20,603		$	3,918
Remediation expense	307			556			18,591
Remediation payments	(1,416		)	(2,983		)	(1,906		)
Balance at end of the period	$	17,067		$	18,176		$	20,603