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]	Accumulated Other Comprehensive (Loss) Income [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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(9,622)					(9,622)
Unrealized gain on short-term investments	294				294
Changes in pension plan assets and benefit obligations	2				2
Amortization of unearned stock-based compensation	4,334			4,334
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		128
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(531)	$ (3)		(528)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, Shares		(28)
Retirement of treasury stock, shares		(8)
Balance at Sep. 30, 2016	621,487	$ 1,278	$ 291	308,690	(271)	311,499
Balance, Shares at Sep. 30, 2016		12,777	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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(21,798)					(21,798)
Unrealized gain on short-term investments	235				235
Changes in pension plan assets and benefit obligations	(1)				(1)
Amortization of unearned stock-based compensation	3,074			3,074
Issuance of non-voting common stock (upon vesting of restricted stock units)	3	$ 13		(10)
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		136
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(258)	$ (2)		(256)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, Shares		(22)
Cumulative effect adjustment of unrecognized tax benefits	1,028					1,028
Balance at Sep. 30, 2017	$ 582,061	$ 1,289	$ 291	$ 307,054	$ (244)	$ 273,671
Balance, Shares at Sep. 30, 2017		12,893	2,906