Accumulated Other Comprehensive Loss (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Components of comprehensive income
Net income	$ 3,792	$ 3,947	$ 4,060	$ 4,624	$ 4,515	$ 7,062	$ 5,795	$ 4,813	$ 16,423	$ 22,186	$ 18,040
Minimum pension liability adjustments									100	(234)	47
Reclassification to interest expense									1,320	1,587	1,805
Deferred income tax (expense) benefit									(348)	(4)	165
Mark-to-market of interest rate hedge									(463)	(1,341)	(2,308)
Comprehensive income									17,032	22,194	17,749
Accumulated other comprehensive loss
Mark-to-market for interest rate hedges, net of tax benefit	(1,674)				(2,219)				(1,674)	(2,219)
Minimum pension liability adjustments, net of tax benefit	(718)				(782)				(718)	(782)
Accumulated other comprehensive loss	$ (2,392)				$ (3,001)				$ (2,392)	$ (3,001)