Consolidated Statements of Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance, beginning of period at Dec. 31, 2010	$ 204,439	$ 40,033	$ 330,894	$ (52,733)	$ (113,755)
Balance, beginning of period (shares) at Dec. 31, 2010		40,032,587
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options and stock awards (shares)	77,446	77,446
Exercise of stock options and stock awards	1,338	77	1,261	0	0
Performance share and restricted stock unit awards (shares)		92,137
Performance share and restricted stock unit awards	2,836	92	2,744	0	0
Net income	40,266	0	0	40,266	0
Pension plans and OPEB obligations	(27,127)	0	0	0	(27,127)
Transfer of assets from REIT to subsidiary	(5,693)	0	(5,693)	0	0
Common distributions, $1.84 per share 2011, $1.24 per share 2012, $1.28 per share 2013	(73,921)	0	0	(73,921)	0
Balance, end of period at Dec. 31, 2011	142,138	40,202	329,206	(86,388)	(140,882)
Balance, end of period (shares) at Dec. 31, 2011		40,202,170
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options and stock awards (shares)	60,857	60,857
Exercise of stock options and stock awards	1,092	61	1,031	0	0
Performance share and restricted stock unit awards (shares)		126,153
Performance share and restricted stock unit awards	2,861	126	3,096	(361)	0
Net income	42,594	0	0	42,594	0
Pension plans and OPEB obligations	(16)	0	0	0	(16)
Transfer of assets from REIT to subsidiary	15	0	15	0	0
Common distributions, $1.84 per share 2011, $1.24 per share 2012, $1.28 per share 2013	(50,041)	0	0	(50,041)	0
Balance, end of period at Dec. 31, 2012	138,643	40,389	333,348	(94,196)	(140,898)
Balance, end of period (shares) at Dec. 31, 2012		40,389,180
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, $1.84 per share 2011, $1.24 per share 2012, $1.28 per share 2013	(51,868)	0	0	(51,868)	0
Balance, end of period at Dec. 31, 2013	$ 204,148	$ 40,537	$ 337,887	$ (75,556)	$ (98,720)
Balance, end of period (shares) at Dec. 31, 2013		40,536,879