General - Schedule of Major Streams of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Revenue from contracts with customers	$ 98,443	$ 95,857	$ 174,312	$ 164,424
Alternative revenue programs, net - WCMA	3,933	7,639	3,601	9,915
Other balancing and memorandum accounts revenue, net	(4,611)	(2,750)	(6,447)	(6,111)
Rental income	1,321	1,327	2,662	2,890
Total revenues	99,086	$ 102,073	174,128	$ 171,118
Reduction of revenue related to cost-recovery balancing accounts	$ 1,351		$ 1,030