Goodwill and Related Intangible Assets (Narrative) (Details)		12 Months Ended
	Oct. 31, 2017 reporting_unit	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Goodwill [Line Items]
Goodwill		$ 321,074,000	$ 304,032,000	$ 292,390,000
Accumulated impairment losses		235,400,000	255,530,000
Number of reporting units tested | reporting_unit	11
Goodwill impairment		0	(929,000)
Impairment of indefinite-lived intangibles		247,000	9,048,000	4,863,000
Impairment of definite-lived intangibles		0	198,000	0
Industrial and Infrastructure Products
Goodwill [Line Items]
Impairment of indefinite-lived intangibles		0	7,980,000	4,423,000
Impairment of definite-lived intangibles		0	0	0
Residential Products
Goodwill [Line Items]
Impairment of indefinite-lived intangibles		0	0	440,000
Impairment of definite-lived intangibles		0	$ 0	$ 0
Solar | Residential Products
Goodwill [Line Items]
Goodwill impairment		$ (900,000)