Consolidated statement of changes in stockholders' equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member] Class A Common Stock [Member]	Common Stock [Member] Class B Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning balance at Dec. 31, 2017	$ 58,949	$ 2	$ 4	$ 232,156	$ (168,141)	$ (5,072)
Beginning balance (in shares) at Dec. 31, 2017		26,725	36,508
Net income (loss)	24,684				24,684
Adjustment for acquisitions	(96)			(96)
Exercise of stock options	301	$ 1		300
Exercise of stock options (in shares)		632
Stock-based compensation	216			216
Foreign currency translation, net of tax	1,255					1,255
Retirement related benefit plans, net of tax	(12)					(12)
Ending balance at Mar. 31, 2018	163,041	$ 3	$ 4	232,576	(65,713)	(3,829)
Ending balance (in shares) at Mar. 31, 2018		27,357	36,508
Beginning balance at Dec. 31, 2017	58,949	$ 2	$ 4	232,156	(168,141)	(5,072)
Beginning balance (in shares) at Dec. 31, 2017		26,725	36,508
Net income (loss)	23,604
Foreign currency translation, net of tax	(3,012)
Retirement related benefit plans, net of tax	106
Ending balance at Jun. 30, 2018	294,815	$ 4	$ 3	369,579	(66,793)	(7,978)
Ending balance (in shares) at Jun. 30, 2018		36,518	33,171
Beginning balance at Dec. 31, 2017	58,949	$ 2	$ 4	232,156	(168,141)	(5,072)
Beginning balance (in shares) at Dec. 31, 2017		26,725	36,508
Net income (loss)	15,535
Ending balance at Dec. 31, 2018	293,686	$ 4	$ 3	379,832	(74,863)	(11,290)
Ending balance (in shares) at Dec. 31, 2018		38,349	32,171
Cumulative effect of an accounting change	77,744				77,744
Beginning balance at Mar. 31, 2018	163,041	$ 3	$ 4	232,576	(65,713)	(3,829)
Beginning balance (in shares) at Mar. 31, 2018		27,357	36,508
Net income (loss)	(1,080)				(1,080)
Follow-on public offering, net of offering costs of $370	135,201	$ 1	$ (1)	135,201
Follow-on public offering, net of offering costs of $370 (shares)		5,731	(1,675)
Exercise of stock options	1,368			1,368
Exercise of stock options (in shares)		1,768
Conversion from Class B to Class A common stock		1,662	(1,662)
Stock-based compensation	434			434
Foreign currency translation, net of tax	(4,267)					(4,267)
Retirement related benefit plans, net of tax	118					118
Ending balance at Jun. 30, 2018	294,815	$ 4	$ 3	369,579	(66,793)	(7,978)
Ending balance (in shares) at Jun. 30, 2018		36,518	33,171
Beginning balance at Dec. 31, 2018	293,686	$ 4	$ 3	379,832	(74,863)	(11,290)
Beginning balance (in shares) at Dec. 31, 2018		38,349	32,171
Net income (loss)	13,019				13,019
Exercise of stock options	458			458
Exercise of stock options (in shares)		397
Vesting of restricted stock (in shares)		14
Stock-based compensation	869			869
Foreign currency translation, net of tax	337					337
Retirement related benefit plans, net of tax	228					228
Ending balance at Mar. 31, 2019	308,597	$ 4	$ 3	381,159	(61,844)	(10,725)
Ending balance (in shares) at Mar. 31, 2019		38,760	32,171
Beginning balance at Dec. 31, 2018	293,686	$ 4	$ 3	379,832	(74,863)	(11,290)
Beginning balance (in shares) at Dec. 31, 2018		38,349	32,171
Net income (loss)	9,899
Foreign currency translation, net of tax	1,942
Retirement related benefit plans, net of tax	244
Ending balance at Jun. 30, 2019	359,841	$ 4	$ 3	433,902	(64,964)	(9,104)
Ending balance (in shares) at Jun. 30, 2019		39,672	31,901
Beginning balance at Mar. 31, 2019	308,597	$ 4	$ 3	381,159	(61,844)	(10,725)
Beginning balance (in shares) at Mar. 31, 2019		38,760	32,171
Net income (loss)	(3,120)				(3,120)
Equity component of convertible senior notes, net of issuance costs	50,009			50,009
Exercise of stock options	812			812
Exercise of stock options (in shares)		608
Vesting of restricted stock (in shares)		34
Conversion from Class B to Class A common stock		270	(270)
Stock-based compensation	1,922			1,922
Foreign currency translation, net of tax	1,605					1,605
Retirement related benefit plans, net of tax	16					16
Ending balance at Jun. 30, 2019	$ 359,841	$ 4	$ 3	$ 433,902	$ (64,964)	$ (9,104)
Ending balance (in shares) at Jun. 30, 2019		39,672	31,901