Consolidated Statements of Shareholders' Equity (USD $) In Thousands	Total	Non-Voting Common Stock [Member]	Voting Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2011	$ 477,337	$ 1,246	$ 285	$ 291,403	$ (93)	$ 184,496
Balance, Shares at Dec. 31, 2011		12,459	2,853
Net earnings	18,057					18,057
Unrealized gain loss on short-term investments	55				55
Changes in pension plan assets and benefit obligations	(13)				(13)
Issuance of non-voting common stock (upon vesting of restricted stock units)	4,179			4,179
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		106
Balance at Dec. 31, 2012	499,615	1,246	285	295,582	(51)	202,553
Balance, Shares at Dec. 31, 2012		12,565	2,853
Net earnings	58,956					58,956
Unrealized gain loss on short-term investments	28				28
Changes in pension plan assets and benefit obligations	(1)				(1)
Amortization of unearned stock-based compensation	2,403			2,403
Issuance of non-voting common stock (upon vesting of restricted stock units)	17	17
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		4
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(65)	(6)		(59)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares		(1)
Issuance of voting common stock (upon vesting of restricted stock units)	9		9
Issuance of voting common stock (upon vesting of restricted stock units), shares			84
Cancellation of restricted voting stock units for tax withholdings on vested shares	(997)		(3)	(994)
Cancellation of restricted voting stock units for tax withholdings on vested shares, shares			(31)
Balance at Dec. 31, 2013	559,965	1,257	291	296,932	(24)	261,509
Balance, Shares at Dec. 31, 2013		12,568	2,906
Net earnings	32,688					32,688
Unrealized gain loss on short-term investments	(173)				(173)
Changes in pension plan assets and benefit obligations	10				10
Amortization of unearned stock-based compensation	4,753			4,753
Issuance of non-voting common stock (upon vesting of restricted stock units)	1	1
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		12
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(178)			(178)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares		(4)
Other	2			26	(24)
Balance at Dec. 31, 2014	$ 597,068	$ 1,258	$ 291	$ 301,533	$ (211)	$ 294,197
Balance, Shares at Dec. 31, 2014		12,576	2,906