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	Restricted Stock	Restricted Stock Additional Paid-in Capital	Performance Shares	Performance Shares Common Stock	Performance Shares Additional Paid-in Capital
Balances at beginning of period (in shares) at Dec. 31, 2015		65,828,653				25,384,834
Balances at beginning of period at Dec. 31, 2015	$ 1,016,538	$ 65,829	$ 320,073	$ 1,067,396	$ (13,914)	$ (422,846)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock based compensation, grants of common stock, value							$ 4,252	$ 3,017
Stock based compensation, grants of common stock, treasury stock reissued (in share)							(74,181)
Stock based compensation, grants of common stock, treasury stock reissued, value							$ 1,235
Stock awards withheld for taxes (in shares)		5,723
Stock awards withheld for taxes, value	(267)	$ (6)	(261)
Forfeited restricted common stock (in shares)		(298)				(197)
Forfeited restricted common stock, value	(3)					$ (3)
Deferred taxes on stock incentive plan	(364)		(364)
Compensation paid in shares (in shares)						(5,936)
Compensation paid in shares, value	277		178			$ 99
Net income	96,768			96,768
Other comprehensive income (loss)	6,798				6,798
Dividends declared, cash	(49,603)			(49,603)
Balances at end of period (in shares) at Dec. 31, 2016		65,822,632				25,304,914
Balances at end of period at Dec. 31, 2016	1,074,396	$ 65,823	322,643	1,114,561	(7,116)	$ (421,515)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock based compensation, grants of common stock (in shares)										11,314
Stock based compensation, grants of common stock, value							$ 4,160	2,989	$ 932	$ 11	$ 921
Stock based compensation, grants of common stock, treasury stock reissued (in share)							(70,273)
Stock based compensation, grants of common stock, treasury stock reissued, value							$ 1,171
Stock awards withheld for taxes (in shares)		5,258				8,360
Stock awards withheld for taxes, value	(712)	$ (5)	(568)			$ (139)
Forfeited restricted common stock (in shares)						(4,961)
Forfeited restricted common stock, value	(83)					$ (83)
Compensation paid in shares (in shares)						(3,612)
Compensation paid in shares, value	192		132			$ 60
Net income	98,261			98,261
Other comprehensive income (loss)	18,174				18,174
Dividends declared, cash	$ (53,337)			(53,337)
Balances at end of period (in shares) at Dec. 31, 2017	65,694,829	65,828,688				25,244,350.00
Balances at end of period at Dec. 31, 2017	$ 1,142,165	$ 65,829	326,117	1,159,667	11,058	$ (420,506)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle in period of adoption	$ 182			182
Stock based compensation, grants of common stock, value							$ 4,049	$ 3,010	$ 1,499		$ 360
Stock based compensation, grants of common stock, treasury stock reissued (in share)	(96,783)						(62,348)		(68,379)
Stock based compensation, grants of common stock, treasury stock reissued, value							$ 1,039		$ 1,139
Stock awards withheld for taxes (in shares)						32,594
Stock awards withheld for taxes, value	$ (1,676)		(1,133)			$ (543)
Forfeited restricted common stock (in shares)						(4,727)
Forfeited restricted common stock, value	(78)					$ (78)
Compensation paid in shares (in shares)						(3,377)
Compensation paid in shares, value	182		126			$ 56
Net income	84,315			84,315
Other comprehensive income (loss)	(8,814)				(8,814)
Dividends declared, cash	$ (57,539)			(57,539)
Balances at end of period (in shares) at Dec. 31, 2018	65,707,156	65,828,688				25,147,567
Balances at end of period at Dec. 31, 2018	$ 1,164,103	$ 65,829	$ 328,480	1,227,471	(38,784)	$ (418,893)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle in period of adoption	$ 0			$ 41,028	$ (41,028)