CONSOLIDATED STATEMENTS OF CHANGES IN CAPITAL - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Partners' Capital [Roll Forward]
Beginning balance		$ 312,006		$ 393,879	$ 312,006	$ 393,879	$ 485,671
Net income	$ 18,849	$ 13,583	$ 17,882	$ 15,914	17,135	31,652	38,380
Issuance of common units, net					51,056	(29)	(590)
General partner contribution					1,098		55
Cash distributions					(76,938)	(118,178)	(133,316)
Reimbursement of excess purchase price over carrying value of acquired assets					1,125	4,125	2,250
Excess purchase price over carrying value of acquired assets					(7,887)
Unit-based compensation					650	904	1,429
Purchase of treasury units					(4)	(347)
Ending balance	298,241		312,006		$ 298,241	$ 312,006	$ 393,879
Common | Common
Increase (Decrease) in Partners' Capital [Roll Forward]
Beginning balance (in shares)		35,452,062		35,456,612	35,452,062	35,456,612	35,365,912
Beginning balance		$ 304,594		$ 380,845	$ 304,594	$ 380,845	$ 470,943
Net income					$ 16,792	23,233	22,042
Issuance of common units, net (in shares)					2,990,000
Issuance of common units, net					$ 51,056	$ (29)	$ (590)
Issuance of restricted units (in shares)					12,000	13,800	91,950
Forfeiture of restricted units (in shares)					(9,250)	(2,250)	(1,250)
Cash distributions					$ (75,399)	$ (104,137)	$ (115,229)
Reimbursement of excess purchase price over carrying value of acquired assets					1,125	4,125	2,250
Excess purchase price over carrying value of acquired assets					(7,887)
Unit-based compensation					$ 650	$ 904	1,429
Purchase of treasury units (in shares)					(200)	(16,100)
Purchase of treasury units					$ (4)	$ (347)
Ending balance	$ 290,927		$ 304,594		$ 290,927	$ 304,594	$ 380,845
Ending balance (in shares)	38,444,612		35,452,062		38,444,612	35,452,062	35,456,612
General Partner
Increase (Decrease) in Partners' Capital [Roll Forward]
Beginning balance		$ 7,412		$ 13,034	$ 7,412	$ 13,034	$ 14,728
Net income					343	8,419	16,338
General partner contribution					1,098		55
Cash distributions					(1,539)	(14,041)	(18,087)
Ending balance	$ 7,314		$ 7,412		$ 7,314	$ 7,412	$ 13,034