Consolidated Statement of Member's Equity Consolidated Statement of Member's Equity - USD ($) $ in Thousands	Total	Member's Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance at beginning of period at Dec. 31, 2015		$ 3,176,499	$ 2,379,385	$ (31)
Cash contributions from parent	$ 502,000	502,000
Net income	523,375		523,375
Cash distributions to parent	(440,000)		(440,000)
Equity interest in unrealized gain on interest rate hedge	41			41
Noncash distribution to Parent			0
Balance at end of period at Dec. 31, 2016	6,141,269	3,678,499	2,462,760	10
Cash contributions from parent	410,000	410,000
Net income	55,728		55,728
Cash distributions to parent	(430,000)		(430,000)
Equity interest in unrealized gain on interest rate hedge	327			327
Noncash distribution to Parent			240,000
Balance at end of period at Dec. 31, 2017	5,937,324	4,088,499	1,848,488	337
Cash contributions from parent	340,000	340,000
Net income	741,079		741,079
Cash distributions to parent	(490,000)		(490,000)
Equity interest in unrealized gain on interest rate hedge	197			197
Noncash distribution to Parent			0
Balance at end of period at Dec. 31, 2018	$ 6,528,600	$ 4,428,499	$ 2,099,567	$ 534