Consolidated Statements of Stockholders' Equity (USD $) In Thousands	Total	Common stock [Member]	Additional paid-in capital [Member]	Retained earnings [Member]	Accumulated other comprehensive income [Member]	Treasury stock [Member]
Beginning of year at Dec. 31, 2013		$ 198,240	$ 288,182	$ 1,202,015	$ 24,851	$ (559,360)
Dividend reinvestment plan (shares: 13,533 – 2015; 15,283 - 2014)		31	320
Stock purchase and compensation plans (shares: 469,014 - 2015; 401,179 - 2014)		802	5,863
Net income	17,974us-gaap_NetIncomeLoss			17,974
Dividends to stockholders ($0.14 per share - 2015; $0.13 per share - 2014)				(7,412)
Other comprehensive income	16,678				16,678
Acquisition of treasury stock (shares: 129,257 - 2015; 118,876 - 2014)						(2,653)
End of period at Mar. 31, 2014	1,185,531us-gaap_StockholdersEquity	199,073	294,365	1,212,577	41,529	(562,013)
Beginning of year at Dec. 31, 2014	1,275,586us-gaap_StockholdersEquity	199,896	305,385	1,313,440	19,788	(562,923)
Dividend reinvestment plan (shares: 13,533 – 2015; 15,283 - 2014)		27	346
Stock purchase and compensation plans (shares: 469,014 - 2015; 401,179 - 2014)		938	7,199
Net income	39,708us-gaap_NetIncomeLoss			39,708
Dividends to stockholders ($0.14 per share - 2015; $0.13 per share - 2014)				(8,113)
Other comprehensive income	3,827				3,827
Acquisition of treasury stock (shares: 129,257 - 2015; 118,876 - 2014)						(3,580)
End of period at Mar. 31, 2015	$ 1,315,938us-gaap_StockholdersEquity	$ 200,861	$ 312,930	$ 1,345,035	$ 23,615	$ (566,503)

Amount of paid and unpaid common stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

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