Partners' Capital - Net Income Per Unit (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Partners' Capital Notes [Abstract]
Net income	$ 12,818	$ 13,583
Less general partner’s interest in net income (loss):
Distributions payable on behalf of IDRs	0	0
Distributions payable on behalf of general partner interest	392	392
General partner interest in undistributed loss	(136)	(120)
Less income allocable to unvested restricted units	8	35
Limited partners’ interest in net income (loss)	$ 12,554	$ 13,276
Basic weighted average limited partner units outstanding (in shares)	38,621,375	37,321,263
Dilutive effect of restricted units issued (in shares)	8,252	46,121
Total weighted average limited partner diluted units outstanding (in shares)	38,629,627	37,367,384