CONSOLIDATED STATEMENTS OF CHANGES IN CAPITAL (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Increase (Decrease) in Partners' Capital [Roll Forward]
Balances	$ 274,806	$ 264,951	$ 246,379
Net Income	24,342	16,022	22,203
General partner contribution	1,505	1,089	1,324
Excess purchase price over carrying value of acquired assets	(19,685)	(4,590)
Units issued in connection with Cross acquisition			32,957
Recognition of beneficial conversion feature	0	0	0
Follow-on public offerings	70,330	78,600	20,000
Issuance of common units	70,330	78,600	20,000
Redemption of common units		(28,070)
Cash distributions	(64,497)	(56,696)	(47,462)
Conversion of subordinated units to common units	0		0
Unit-based compensation	190	113	98
Purchase of treasury units	(582)	(108)	(78)
Contributions to parent			(13,329)
Adjustment in fair value of derivatives	(793)	3,495	2,859
Balances	285,616	274,806	264,951
Net income	24,342	16,022	22,203
Parent Net Investment [Member]
Increase (Decrease) in Partners' Capital [Roll Forward]
Balances	0	0	11,665
Net Income	0	0	1,664
General partner contribution	0	0	0
Excess purchase price over carrying value of acquired assets	0	0
Recognition of beneficial conversion feature	0	0	0
Follow-on public offerings	0	0	0
Issuance of common units	0	0	0
Redemption of common units		0
Cash distributions	0	0	0
Conversion of subordinated units to common units	0		0
Unit-based compensation	0	0	0
Purchase of treasury units	0	0	0
Contributions to parent			(13,329)
Adjustment in fair value of derivatives	0	0	0
Balances	0	0	0
Net income	0	0	1,664
Common [Member]
Increase (Decrease) in Partners' Capital [Roll Forward]
Balances (in units)	17,707,832	16,057,832	13,688,152
Balances	250,787	245,683	239,333
Net Income	19,053	12,153	16,310
General partner contribution	0	0	0
Excess purchase price over carrying value of acquired assets	(19,685)	(4,590)
Units issued in connection with Cross acquisition (in units)			804,721
Units issued in connection with Cross acquisition			16,523
Recognition of beneficial conversion feature	(1,108)	(1,108)	(111)
Follow-on public offerings (in units)	1,874,500	2,650,000	714,285
Issuance of common units (in units)	1,874,500	2,650,000	714,285
Follow-on public offerings	70,330	78,600	20,000
Issuance of common units	70,330	78,600	20,000
Redemption of common units (in units)		(1,000,000)
Redemption of common units		(28,070)
Cash distributions	(58,252)	(51,886)	(41,064)
Conversion of subordinated units to common units (in units)	889,444		850,674
Conversion of subordinated units to common units	18,829		(5,328)
Unit-based compensation (in units)	14,850	3,500	3,000
Unit-based compensation	190	113	98
Purchase of treasury units (in units)	(14,850)	(3,500)	(3,000)
Purchase of treasury units	(582)	(108)	(78)
Contributions to parent			0
Adjustment in fair value of derivatives	0	0	0
Balances (in units)	20,471,776	17,707,832	16,057,832
Balances	279,562	250,787	245,683
Net income	19,053	12,153	16,310
Subordinated [Member]
Increase (Decrease) in Partners' Capital [Roll Forward]
Balances (in units)	889,444	889,444	850,674
Balances	17,721	16,613	(3,688)
Net Income	0	0	980
General partner contribution	0	0	0
Excess purchase price over carrying value of acquired assets	0	0
Units issued in connection with Cross acquisition (in units)			889,444
Units issued in connection with Cross acquisition			16,434
Recognition of beneficial conversion feature	1,108	1,108	111
Follow-on public offerings	0	0	0
Issuance of common units	0	0	0
Redemption of common units		0
Cash distributions	0	0	(2,552)
Conversion of subordinated units to common units (in units)	(889,444)		(850,674)
Conversion of subordinated units to common units	(18,829)		5,328
Unit-based compensation	0	0	0
Purchase of treasury units	0	0	0
Contributions to parent			0
Adjustment in fair value of derivatives	0	0	0
Balances (in units)		889,444	889,444
Balances	0	17,721	16,613
Net income	0	0	980
General Partner [Member]
Increase (Decrease) in Partners' Capital [Roll Forward]
Balances	4,879	4,731	4,004
Net Income	5,289	3,869	3,249
General partner contribution	1,505	1,089	1,324
Excess purchase price over carrying value of acquired assets	0	0
Units issued in connection with Cross acquisition			0
Recognition of beneficial conversion feature	0	0	0
Follow-on public offerings	0	0	0
Issuance of common units	0	0	0
Redemption of common units		0
Cash distributions	(6,245)	(4,810)	(3,846)
Conversion of subordinated units to common units	0		0
Unit-based compensation	0	0	0
Purchase of treasury units	0	0	0
Contributions to parent			0
Adjustment in fair value of derivatives	0	0	0
Balances	5,428	4,879	4,731
Net income	5,289	3,869	3,249
Accumulated Other Comprehensive Income (Loss) [Member]
Increase (Decrease) in Partners' Capital [Roll Forward]
Balances	1,419	(2,076)	(4,935)
Net Income	0	0	0
General partner contribution	0	0	0
Excess purchase price over carrying value of acquired assets	0	0
Units issued in connection with Cross acquisition			0
Recognition of beneficial conversion feature	0	0	0
Follow-on public offerings	0	0	0
Issuance of common units	0	0	0
Redemption of common units		0
Cash distributions	0	0	0
Conversion of subordinated units to common units	0		0
Unit-based compensation	0	0	0
Purchase of treasury units	0	0	0
Contributions to parent			0
Adjustment in fair value of derivatives	(793)	3,495	2,859
Balances	626	1,419	(2,076)
Net income	$ 0	$ 0	$ 0