Stockholders' Equity - Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			$ 979,069				$ 979,069
Net transfer to Dover				$ 2,118	$ (739,284)	$ (1,635)
Other comprehensive loss	$ (2,832)	$ 1,042	1,189	(76)	(6,658)	(1,642)	(601)	$ (8,376)
Ending balance	1,038,801						1,038,801
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(40,675)	(41,717)	(42,906)	(37,481)	(29,373)	(26,416)	(42,906)	(26,416)
Reclassification adjustment for cumulative effect of change in accounting principle									$ (1,315)
Net transfer to Dover				(970)	(1,450)
Other comprehensive income (loss) before reclassifications, net of tax	(2,900)	974	767	(515)	(6,704)	(1,691)
Reclassification adjustment for net losses included in net income, net of tax	68	68	422	439	46	49
Other comprehensive loss	(2,832)	1,042	1,189	(76)	(6,658)	(1,642)
Ending balance	(43,507)	(40,675)	(41,717)	(38,527)	(37,481)	(29,373)	(43,507)	(38,527)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(34,082)	(35,056)	(36,146)	(30,501)	(23,627)	(21,936)	(36,146)	(21,936)
Other comprehensive income (loss) before reclassifications, net of tax	(2,900)	974	1,090	(515)	(6,874)	(1,691)
Other comprehensive loss	(2,900)	974	1,090	(515)	(6,874)	(1,691)
Ending balance	(36,982)	(34,082)	(35,056)	(31,016)	(30,501)	(23,627)	(36,982)	(31,016)
Defined Pension and Other Post-Retirement Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(6,593)	(6,661)	(6,760)	(6,980)	(5,746)	(4,480)	(6,760)	(4,480)
Reclassification adjustment for cumulative effect of change in accounting principle									$ (1,315)
Net transfer to Dover				(970)	(1,450)
Other comprehensive income (loss) before reclassifications, net of tax			(323)		170
Reclassification adjustment for net losses included in net income, net of tax	68	68	422	439	46	49
Other comprehensive loss	68	68	99	439	216	49
Ending balance	$ (6,525)	$ (6,593)	$ (6,661)	$ (7,511)	$ (6,980)	$ (5,746)	$ (6,525)	$ (7,511)