Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended													12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Restructuring expenses														$ 625	$ 1,431	$ 1,361
Revenue	$ 494,784		$ 464,712		$ 434,124		$ 406,814		$ 474,738		$ 405,228	$ 402,729	$ 379,472	1,800,434	1,662,167	1,557,067
Gross profit	237,832		222,891		206,732		191,321	[1]	214,733	[2]	191,376	183,322	177,690	858,776	767,121	753,561
Net earnings	$ (50,290)	[3]	$ 63,529	[3]	$ 51,413	[3]	$ 42,571	[3]	$ 61,500	[4]	$ 58,633	$ 45,368	$ 1,125	$ 107,223	$ 166,626	$ 241,686
Basic earnings per share:
Basic earnings per share (in dollars per share)	$ (0.36)		$ 0.46		$ 0.38		$ 0.31		$ 0.45		$ 0.43	$ 0.33	$ 0.01	$ 0.78	$ 1.22	$ 1.73
Diluted earnings per share:
Diluted earnings per share (in dollars per share)	$ (0.36)		$ 0.46		$ 0.37		$ 0.31		$ 0.45		$ 0.43	$ 0.33	$ 0.01	$ 0.77	$ 1.20	$ 1.72

[1] (1)Net earnings for the fourth quarter of 2017 includes a discrete tax charge of $94.4 million associated with U.S. Tax Cuts and Jobs Act enacted in December 2017 and a loss on net assets held for sale of $23.6 million.

[2] .

[3] (2)Net earnings for the first quarter of 2016 includes a discrete tax charge for certain tax legislation in Belgium of $39.6 million.

[4] .