Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Research expense													$ 26,900	$ 29,800	$ 30,700
Defined benefit pension plan assets	$ 21,300				$ 14,200								21,300	14,200
Gross profit	$ 96,600	$ 104,100	$ 105,200	$ 91,800	87,900	$ 92,400	$ 91,700	$ 77,700	$ 77,500	$ 79,600	$ 63,200	$ 76,000	$ 397,701	349,749	$ 296,283
Interest rate on contract receivables	2.00%												2.00%
Extended payment term of receivables													over a 10-year period starting in 2020
Long term contract [Member]
Gross profit	$ 10,800				2,000				$ 11,500
Financial Assets [Member]
Defined benefit pension plan assets	800				5,300								$ 800	5,300
Russia Entity [Member]
Equity method investment	$ 500				$ 400								$ 500	$ 400