Acquisitions (Summary of the Results of Operations of the Acquired Assets for the Period of Common Control) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Operations and maintenance (excluding depreciation and amortization reflected below)										$ 33,501	$ 34,211	$ 29,601	[1]
Depreciation and amortization										5,111	5,647	5,828	[1],[2]
Total operating expenses		$ 10,635	$ 10,548	$ 10,655	$ 10,997	$ 10,693	$ 11,474	$ 11,043	$ 11,071	$ 42,835	44,281	38,543	[1]
Net loss attributable to sponsor	[1]											$ (273)
Assets of Hereford, Texas and Hopewell, Virginia [Member]
Business Acquisition [Line Items]
Operations and maintenance (excluding depreciation and amortization reflected below)											232
Depreciation and amortization											41
Total operating expenses											273
Net loss attributable to sponsor											$ (273)

[1] Recast to include historical results of operations related to net assets acquired in a transfer between entities under common control. See Notes 1 and 4 in the accompanying notes to consolidated financial statements for further discussion.

[2] Recast to include historical cash flow activity related to net assets acquired in a transfer between entities under common control. See Notes 1 and 4 in the accompanying notes to consolidated financial statements for further discussion.