Consolidated Condensed Statements of Stockholders' Equity (Unaudited) (USD $) Share data in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2013	$ 333,000	$ 85,077,000	$ 273,727,000	$ 359,137,000
Balance (in shares) at Dec. 31, 2013	33,301
Net income			5,287,000	5,287,000
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	48
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards		370,000		370,000
Tax benefits from share-based payment arrangement exercises		50,000		50,000
Share-based payment arrangement compensation expense		211,000		211,000
Dividends on common stock			(834,000)	(834,000)
Balance at Mar. 31, 2014	333,000	85,708,000	278,180,000	364,221,000
Balance (in shares) at Mar. 31, 2014	33,349
Net income			24,547,000	24,547,000
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	70
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	1,000	843,000		844,000
Tax benefits from share-based payment arrangement exercises		109,000		109,000
Share-based payment arrangement compensation expense		710,000		710,000
Dividends on common stock			(2,505,000)	(2,505,000)
Balance at Dec. 31, 2014	334,000	87,370,000	300,222,000	387,926,000
Balance (in shares) at Dec. 31, 2014	33,419
Net income			10,188,000	10,188,000
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	84
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	1,000	972,000		973,000
Tax benefits from share-based payment arrangement exercises		178,000		178,000
Share-based payment arrangement compensation expense		273,000		273,000
Dividends on common stock			(837,000)	(837,000)
Balance at Mar. 31, 2015	$ 335,000	$ 88,793,000	$ 309,573,000	$ 398,701,000
Balance (in shares) at Mar. 31, 2015	33,503