Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Balance, beginning of period at Dec. 31, 2014	$ 225,066	$ 40,605	$ 346,441	$ (43,588)	$ (118,392)
Balance, beginning of period (shares) at Dec. 31, 2014		40,605,179
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Performance shares and restricted stock units	3,390	$ 76	3,406	(92)
Performance share and restricted stock unit (shares)		75,534
Net income	31,714			31,714
Director deferred stock awards	701		701
Pension plans and OPEB obligations	3,889				3,889
Transfer of assets from REIT to subsidiary	(7)		(7)
Cash flow hedge, net of tax of $3, $448, and $-	0
Common distributions, $1.50 per share 2015, $1.50 per share 2016, $1.525 per share 2017	(61,017)			(61,017)
Balance, end of period at Dec. 31, 2015	203,736	$ 40,681	350,541	(72,983)	(114,503)
Balance, end of period (shares) at Dec. 31, 2015		40,680,713
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Performance shares and restricted stock units	3,971	$ 8	4,065	(102)
Performance share and restricted stock unit (shares)		8,263
Net income	10,938			10,938
Director deferred stock awards	668		668
Pension plans and OPEB obligations	3,058				3,058
Repurchase of common stock	(5,956)	$ (170)		(5,786)
Repurchase of common stock (shares)		(169,625)
Cash flow hedge, net of tax of $3, $448, and $-	701				701
Common distributions, $1.50 per share 2015, $1.50 per share 2016, $1.525 per share 2017	(60,842)			(60,842)
Balance, end of period at Dec. 31, 2016	156,274	$ 40,519	355,274	(128,775)	(110,744)
Balance, end of period (shares) at Dec. 31, 2016		40,519,351
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Performance shares and restricted stock units	3,435	$ 93	3,452	(110)
Performance share and restricted stock unit (shares)		92,640
Net income	86,453			86,453
Director deferred stock awards	418		418
Pension plans and OPEB obligations	15,889				15,889
Repurchase of common stock (shares)		0
Cash flow hedge, net of tax of $3, $448, and $-	4				4
Common distributions, $1.50 per share 2015, $1.50 per share 2016, $1.525 per share 2017	(61,931)			(61,931)
Balance, end of period at Dec. 31, 2017	$ 200,542	$ 40,612	$ 359,144	$ (104,363)	$ (94,851)
Balance, end of period (shares) at Dec. 31, 2017		40,611,991