Distributions and Net Income Per Limited Partner Unit (Basic and Diluted Income Per Limited Partner Unit) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Limited Partners' Capital Account [Line Items]
Distribution									$ 64,718	$ 54,249	$ 45,708
(Distribution in excess of net income) net income in excess of distribution									(3,737)	(4,902)	2,962
Limited partners' interest in net income subsequent to IPO	$ 17,371	$ 14,922	$ 13,733	$ 14,955	$ 11,883	$ 12,412	$ 12,757	$ 12,295	$ 60,981	$ 49,347	$ 48,670
Weighted average units outsanding:
Basic and diluted (units)									34,488,058	32,240,858	28,392,470
IDR Holders [Member]
Limited Partners' Capital Account [Line Items]
Distributions declared with respect to the incentive distribution rights									$ 0	$ 733	$ 1,666
(Distribution in excess of net income) net income in excess of distribution									0	0	0
Limited partners' interest in net income subsequent to IPO									0	733	1,666
Limited Partners' Common Units [Member]
Limited Partners' Capital Account [Line Items]
Distribution									64,718	53,516	34,909
(Distribution in excess of net income) net income in excess of distribution									(3,737)	(4,902)	3,101
Limited partners' interest in net income subsequent to IPO									$ 60,981	$ 48,614	$ 38,010
Weighted average units outsanding:
Basic and diluted (units)	35,191,487	35,188,189	35,188,189	32,345,398	32,247,371	32,242,210	32,237,266	32,237,266	34,488,058	32,240,858	22,101,547
Net Income (Loss), Per Outstanding Limited Partnership Unit, Basic, Net of Tax									$ 1.77	$ 1.51	$ 1.72
Limited Partners' Subordinated Units [Member]
Limited Partners' Capital Account [Line Items]
Distribution									$ 0	$ 0	$ 9,133
(Distribution in excess of net income) net income in excess of distribution											(139)
Limited partners' interest in net income subsequent to IPO											$ 8,994
Weighted average units outsanding:
Basic and diluted (units)											6,290,923
Net Income (Loss), Per Outstanding Limited Partnership Unit, Basic, Net of Tax									$ 0	$ 0	$ 1.43