Condensed Consolidated Statement of Member's Equity (Unaudited) - USD ($) $ in Thousands	Total	Member Units [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]
Balance at beginning of period at Dec. 31, 2017		$ 4,088,499	$ 1,848,488	$ 337
Cash contributions from parent	$ 340,000	340,000
Net Income	179,916		179,916
Cash distributions to parent	(55,000)		(55,000)
Equity interest in unrealized gain (loss) on interest rate hedge	405			405
Balance at end of period at Mar. 31, 2018	6,402,645	4,428,499	1,973,404	742
Balance at beginning of period at Dec. 31, 2018	6,528,600	4,428,499	2,099,567	534
Cash contributions from parent	0	0
Net Income	229,702		229,702
Cash distributions to parent	(176,000)		(176,000)
Equity interest in unrealized gain (loss) on interest rate hedge	(230)			(230)
Balance at end of period at Mar. 31, 2019	$ 6,582,072	$ 4,428,499	$ 2,153,269	$ 304