Consolidated Statements of Changes in Member's Equity - USD ($) $ in Thousands	Total	Capital Distributions [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2017	$ 397,828	$ (98,268)	$ 496,096
Net income	49,508		49,508
Dividend payments to related party on preferred members' interest	(4,500)		(4,500)
Capital contributions from parent	158,000	158,000
Capital distributions to parent	(500)	(500)
Revenue recognition adoption (Note 12)	(1,231)		(1,231)
Other	34	34
Balance at Mar. 31, 2018	599,139	59,266	539,873
Balance at Dec. 31, 2018	773,909	99,358	674,551
Net income	59,117		59,117
Dividend payments to related party on preferred members' interest	(4,500)		(4,500)
Capital contributions from parent	110,950	110,950
Capital distributions to parent	(49,800)	(49,800)
Other	27	27
Balance at Mar. 31, 2019	$ 889,703	$ 160,535	$ 729,168