Segment Reporting - Reconciliation of Operating Profit (Loss) from Segments to Consolidated (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated
Net operating income	$ 13,195,000	$ 7,314,000	$ 24,415,000	$ 12,383,000
General and administrative	(6,579,000)	(5,186,000)	(12,355,000)	(10,175,000)
Depreciation and amortization	(17,944,000)	(14,382,000)	(34,612,000)	(32,813,000)
Interest expense	(6,445,000)	(5,762,000)	(12,969,000)	(11,354,000)
Interest income	11,000	90,000	20,000	240,000
Acquisition-related expenses	0	(509,000)	(105,000)	(509,000)
Other expense	(12,000)	(9,000)	(13,000)	(54,000)
Income from continuing operations	6,749,000	1,124,000	11,348,000	706,000
Intersegment sales			0	0
Intersegment transfers			0	0
Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated
Net operating income			$ 71,382,000	$ 55,371,000