Condensed Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Non-Voting Common Stock [Member]	Voting Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2015	$ 626,998	$ 1,269	$ 291	$ 304,884	$ (567)	$ 321,121
Balance, Shares at Dec. 31, 2015		12,685	2,906
Net loss	(8,932)					(8,932)
Unrealized gain on short-term investments	476				476
Changes in pension plan assets and benefit obligations	1				1
Amortization of unearned stock-based compensation	1,485			1,485
Issuance of non-voting common stock (upon vesting of restricted stock units)	12	$ 12
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		121
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(503)	$ (3)		(500)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, Shares		(27)
Retirement of treasury stock	0	$ 0	$ 0	0	0	0
Retirement of treasury stock, shares		(8)
Balance at Mar. 31, 2016	619,537	$ 1,278	$ 291	305,869	(90)	312,189
Balance, Shares at Mar. 31, 2016		12,771	2,906
Balance at Dec. 31, 2016	599,778	$ 1,278	$ 291	304,246	(478)	294,441
Balance, Shares at Dec. 31, 2016		12,779	2,906
Net loss	(15,248)					(15,248)
Unrealized gain on short-term investments	104				104
Changes in pension plan assets and benefit obligations	(1)				(1)
Amortization of unearned stock-based compensation	552			552
Issuance of non-voting common stock (upon vesting of restricted stock units)	2	$ 2
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		27
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(101)	$ (1)		(100)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, Shares		(9)
Cumulative effect adjustment of unrecognized tax benefits	1,028					1,028
Balance at Mar. 31, 2017	$ 586,114	$ 1,279	$ 291	$ 304,698	$ (375)	$ 280,221
Balance, Shares at Mar. 31, 2017		12,797	2,906