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	(4,655)					(4,655)
Unrealized gain on short-term investments	611				611
Changes in pension plan assets and benefit obligations	1				1
Amortization of unearned stock-based compensation	2,882			2,882
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, shares		(8)
Balance at Jun. 30, 2016	625,346	$ 1,278	$ 291	307,266	45	316,466
Balance, Shares at Jun. 30, 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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(18,521)					(18,521)
Unrealized gain on short-term investments	210				210
Changes in pension plan assets and benefit obligations	14				14
Amortization of unearned stock-based compensation	1,816			1,816
Issuance of non-voting common stock (upon vesting of restricted stock units)	13	$ 13
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		134
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(277)	$ (2)		(275)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, Shares		(21)
Cumulative effect adjustment of unrecognized tax benefits	1,028					1,028
Balance at Jun. 30, 2017	$ 584,061	$ 1,289	$ 291	$ 305,787	$ (254)	$ 276,948
Balance, Shares at Jun. 30, 2017		12,892	2,906