STATEMENTS OF CHANGES IN COMMON STOCK EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss), Net of Tax	Treasury Stock
Balances at beginning of period (in shares) at Dec. 31, 2012		65,604,997				25,492,919
Balances at beginning of period at Dec. 31, 2012	$ 824,999	$ 65,605	$ 310,994	$ 939,131	$ (66,084)	$ (424,647)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Restricted common stock grants and deferred compensation (in shares)		96,279
Restricted common stock grants and deferred compensation	2,798	$ 96	2,702
Performance share awards vested (in shares)		64,275
Performance share awards vested	849	$ 64	785
Stock awards withheld for taxes (in shares)		(23,808)
Stock awards withheld for taxes	(811)	$ (23)	$ (788)
Forfeited restricted common stock (in shares)		(1,549)
Forfeited restricted common stock	(1)	$ (1)
Deferred taxes on stock incentive plan	427		$ 427
Stock options exercised, in shares		15,000
Stock options exercised	192	$ 15	177
Grants of share based compensation (in shares)		4,431
Grants of share based compensation	150	$ 4	146
Net income	88,583			88,583
Other comprehensive income (loss)	68,696				68,696
Dividends, Common Stock, Cash	(42,049)			(42,049)
Balances at end of period (in shares) at Dec. 31, 2013		65,759,625.000				25,492,919.000
Balances at end of period at Dec. 31, 2013	943,833	$ 65,760	314,443	985,665	2,612	$ (424,647)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Restricted common stock grants and deferred compensation (in shares)		103,672
Restricted common stock grants and deferred compensation	4,279	$ 104	4,175
Stock awards withheld for taxes (in shares)		(4,696)
Stock awards withheld for taxes	(188)	$ (5)	(183)
Forfeited restricted common stock (in shares)		(19,162)
Forfeited restricted common stock	(19)	$ (19)
Deferred taxes on stock incentive plan	(302)		(302)
Grants of share based compensation (in shares)		10,104
Grants of share based compensation	392	$ 10	382
Net income	91,428			91,428
Other comprehensive income (loss)	(10,613)				(10,613)
Dividends, Common Stock, Cash	$ (44,556)			(44,556)
Balances at end of period (in shares) at Dec. 31, 2014	65,725,246	65,849,543				25,492,919
Balances at end of period at Dec. 31, 2014	$ 984,254	$ 65,850	318,515	1,032,537	(8,001)	$ (424,647)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Restricted common stock grants and deferred compensation (in shares)		6,356
Restricted common stock grants and deferred compensation	$ 3,829	$ 6	2,266
Stock Issued During Period, Shares, Treasury Stock Reissued	(108,085)					(93,455)
Stock Issued During Period, Value, Treasury Stock Reissued						$ 1,557
Stock awards withheld for taxes (in shares)		(15,031)
Stock awards withheld for taxes	$ (571)	$ (15)	(556)
Forfeited restricted common stock (in shares)		(12,215)				(871)
Forfeited restricted common stock	(26)	$ (12)				$ (14)
Deferred taxes on stock incentive plan	(475)		(475)
Grants of share based compensation (in shares)						15,501
Grants of share based compensation	581		323			$ 258
Net income	81,918			81,918
Other comprehensive income (loss)	(5,913)				(5,913)
Dividends, Common Stock, Cash	$ (47,059)			(47,059)
Balances at end of period (in shares) at Dec. 31, 2015	65,709,819	65,828,653				25,384,834
Balances at end of period at Dec. 31, 2015	$ 1,016,538	$ 65,829	$ 320,073	$ 1,067,396	$ (13,914)	$ (422,846)