CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common Stock	Capital Stock Expense	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2016	$ 1,559,035	$ 1,296,539	$ (6,357)	$ 273,408	$ (4,555)
Increase (Decrease) in Stockholder's Equity [Roll Forward]
Net Income	176,668			176,668
Other Comprehensive (Loss) Income, Net of Tax	(1,671)				(1,671)
Dividends Declared to Parent	(70,000)			(70,000)
Ending Balances at Dec. 31, 2017	1,664,032	1,296,539	(6,357)	380,076	(6,226)
Increase (Decrease) in Stockholder's Equity [Roll Forward]
Net Income	188,323			188,323
Other Comprehensive (Loss) Income, Net of Tax	2,390				2,390
Dividends Declared to Parent	(85,000)			(85,000)
Contribution from Parent	50,000	50,000
Adoption of ASU, Cumulative Effect Adjustment	0			878	(878)
Ending Balances at Dec. 31, 2018	1,819,745	1,346,539	(6,357)	484,277	(4,714)
Increase (Decrease) in Stockholder's Equity [Roll Forward]
Net Income	186,515			186,515
Other Comprehensive (Loss) Income, Net of Tax	(3,057)				(3,057)
Dividends Declared to Parent	(75,000)			(75,000)
Contribution from Parent	50,000	50,000
Ending Balances at Dec. 31, 2019	$ 1,978,203	$ 1,396,539	$ (6,357)	$ 595,792	$ (7,771)