Consolidated Statements of Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Stockholders' Equity Attributable to Parent	$ 138,643	$ 142,138	$ 138,643	$ 142,138	$ 204,439	$ 229,790
Exercise of stock options and stock awards (shares)			60,857	77,446	107,296
Exercise of stock options and stock awards			1,092	1,338	2,156
Performance share and restricted stock unit awards			2,861	2,836	1,877
Net income	13,864	(1,478)	42,594	40,266	40,394
Pension plans and OPEB obligations			(16)	(27,127)	13,722
Transfer of assets from REIT to subsidiary			15	(5,693)	(1,922)
Common distributions, $2.04 per share 2010, $1.84 per share 2011, $1.24 per share 2012			(50,041)	(73,921)	(81,578)
Common Stock [Member]
Common Stock, Shares, Outstanding	40,389,180	40,202,170	40,389,180	40,202,170	40,032,587	39,826,784
Stockholders' Equity Attributable to Parent	40,389	40,202	40,389	40,202	40,033	39,827
Exercise of stock options and stock awards (shares)			60,857	77,446	107,296
Exercise of stock options and stock awards			61	77	107
Performance share and restricted stock unit awards (shares)			126,153	92,137	98,507
Performance share and restricted stock unit awards			126	92	99
Net income			0	0	0
Pension plans and OPEB obligations			0	0	0
Transfer of assets from REIT to subsidiary			0	0	0
Common distributions, $2.04 per share 2010, $1.84 per share 2011, $1.24 per share 2012			0	0	0
Additional Paid-in Capital [Member]
Stockholders' Equity Attributable to Parent	333,348	329,206	333,348	329,206	330,894	328,989
Exercise of stock options and stock awards			1,031	1,261	2,049
Performance share and restricted stock unit awards			3,096	2,744	1,778
Net income			0	0	0
Pension plans and OPEB obligations			0	0	0
Transfer of assets from REIT to subsidiary			15	(5,693)	(1,922)
Common distributions, $2.04 per share 2010, $1.84 per share 2011, $1.24 per share 2012			0	0	0
Retained Earnings [Member]
Stockholders' Equity Attributable to Parent	(94,196)	(86,388)	(94,196)	(86,388)	(52,733)	(11,549)
Exercise of stock options and stock awards			0	0	0
Performance share and restricted stock unit awards			(361)	0	0
Net income			42,594	40,266	40,394
Pension plans and OPEB obligations			0	0	0
Transfer of assets from REIT to subsidiary			0	0	0
Common distributions, $2.04 per share 2010, $1.84 per share 2011, $1.24 per share 2012			(50,041)	(73,921)	(81,578)
Accumulated Other Comprehensive Income (Loss) [Member]
Stockholders' Equity Attributable to Parent	(140,898)	(140,882)	(140,898)	(140,882)	(113,755)	(127,477)
Exercise of stock options and stock awards			0	0	0
Performance share and restricted stock unit awards			0	0	0
Net income			0	0	0
Pension plans and OPEB obligations			(16)	(27,127)	13,722
Transfer of assets from REIT to subsidiary			0	0	0
Common distributions, $2.04 per share 2010, $1.84 per share 2011, $1.24 per share 2012			$ 0	$ 0	$ 0