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 (shares) at Dec. 31, 2012		40,389,180
Balance, beginning of period at Dec. 31, 2012	$ 138,643	$ 40,389	$ 333,348	$ (94,196)	$ (140,898)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options and stock awards (shares)	70,968	70,968
Exercise of stock options and stock awards	$ 1,904	$ 71	1,833	0	0
Performance share and restricted stock unit awards (shares)		76,731
Performance share and restricted stock unit awards	2,710	$ 77	2,706	(73)	0
Net income	70,581	0	0	70,581	0
Pension plans and OPEB obligations	42,178	0	0	0	42,178
Common distributions, $1.28 per share 2013, $1.425 per share 2014, $1.50 per share 2015	(51,868)	$ 0	0	(51,868)	0
Balance, end of period (shares) at Dec. 31, 2013		40,536,879.000
Balance, end of period at Dec. 31, 2013	$ 204,148	$ 40,537	337,887	(75,556)	(98,720)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options and stock awards (shares)	12,859	12,859
Exercise of stock options and stock awards	$ 398	$ 13	385	0	0
Performance share and restricted stock unit awards (shares)		55,441
Performance share and restricted stock unit awards	2,935	$ 55	2,974	(94)	0
Net income	89,910	0	0	89,910	0
Director deferred stock awards	5,195	0	5,195	0	0
Pension plans and OPEB obligations	(19,672)	0	0	0	(19,672)
Common distributions, $1.28 per share 2013, $1.425 per share 2014, $1.50 per share 2015	(57,848)	$ 0	0	(57,848)	0
Balance, end of period (shares) at Dec. 31, 2014		40,605,179.000
Balance, end of period at Dec. 31, 2014	225,066	$ 40,605	346,441	(43,588)	(118,392)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Performance share and restricted stock unit awards (shares)		75,534
Performance share and restricted stock unit awards	3,390	$ 76	3,406	(92)	0
Net income	31,714	0	0	31,714	0
Director deferred stock awards	701	0	701	0	0
Pension plans and OPEB obligations	3,889	0	0	0	3,889
Transfer of assets from REIT to subsidiary	(7)	0	(7)	0	0
Common distributions, $1.28 per share 2013, $1.425 per share 2014, $1.50 per share 2015	(61,017)	$ 0	0	(61,017)	0
Balance, end of period (shares) at Dec. 31, 2015		40,680,713.000
Balance, end of period at Dec. 31, 2015	$ 203,736	$ 40,681	$ 350,541	$ (72,983)	$ (114,503)