Condensed Consolidated Statement Of Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Common Stock [Member]	Capital In Excess Of Par Value [Member]	Cumulative Dividends In Excess Of Net Income [Member]	Unrealized Gains On Marketable Securities [Member]	Total
Balance, value at Dec. 31, 2008	$ 276	$ 458,911	$ (40,360)	$ 10,788	$ 429,615
Balance, shares at Dec. 31, 2008	27,580,319
Comprehensive income:
Net income			64,229		64,229
Other Comprehensive income:
Unrealized holding gain/(loss) arising during the period				4,462	4,462
Less: reclassification adjustment for gains included in net income				(1,147)	(1,147)
Net gain recognized in other comprehensive income				3,315	3,315
Total comprehensive income					67,544
Shares issued on stock options exercised, shares	49,186
Shares issued on stock options exercised, value		78			78
Share-based compensation		853			853
Cash Dividends:
Cash dividends to common stockholders			(63,478)		(63,478)
Balance, value at Dec. 31, 2009	276	459,842	(39,609)	14,103	434,612
Balance, shares at Dec. 31, 2009	27,629,505
Comprehensive income:
Net income			69,421		69,421
Other Comprehensive income:
Unrealized holding gain/(loss) arising during the period				1,229	1,229
Less: reclassification adjustment for gains included in net income				(2)	(2)
Net gain recognized in other comprehensive income				1,227	1,227
Total comprehensive income					70,648
Shares issued on stock options exercised, shares	59,887
Shares issued on stock options exercised, value	1	182			183
Share-based compensation		2,368			2,368
Cash Dividends:
Cash dividends to common stockholders			(65,311)		(65,311)
Balance, value at Dec. 31, 2010	277	462,392	(35,499)	15,330	442,500
Balance, shares at Dec. 31, 2010	27,689,392				27,689,392
Comprehensive income:
Net income			81,132		81,132
Other Comprehensive income:
Unrealized holding gain/(loss) arising during the period				1,750	1,750
Less: reclassification adjustment for gains included in net income				(9,899)	(9,899)
Net gain recognized in other comprehensive income				(8,149)	(8,149)
Total comprehensive income					72,983
Shares issued on stock options exercised, shares	61,816
Shares issued on stock options exercised, value	1	199			200
Share-based compensation		3,087			3,087
Cash Dividends:
Cash dividends to common stockholders			(75,285)		(75,285)
Balance, value at Dec. 31, 2011	$ 278	$ 465,678	$ (29,652)	$ 7,181	$ 443,485
Balance, shares at Dec. 31, 2011	27,751,208				27,751,208