CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common Stock	Capital Stock Expense	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Loss	Accumulated Other Comprehensive Loss Cumulative Effect, Period of Adoption, Adjustment
Beginning Balance at Dec. 31, 2017	$ 1,664,032	$ 1,296,539	$ (6,357)	$ 380,076	$ 878	$ (6,226)	$ (878)
Increase (Decrease) in Stockholder's Equity [Roll Forward]
Net Income	188,323			188,323
Other Comprehensive Income (Loss), Net of Tax	2,390					2,390
Dividends Declared to Parent	(85,000)			(85,000)
Contributions from Parent	50,000	50,000
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 Income (Loss), Net of Tax	(3,057)					(3,057)
Dividends Declared to Parent	(75,000)			(75,000)
Contributions from Parent	50,000	50,000
Ending Balances at Dec. 31, 2019	1,978,203	1,396,539	(6,357)	595,792		(7,771)
Increase (Decrease) in Stockholder's Equity [Roll Forward]
Net Income	191,405			191,405
Other Comprehensive Income (Loss), Net of Tax	(3,171)					(3,171)
Dividends Declared to Parent	(75,000)			(75,000)
Contributions from Parent	250,000	250,000
Ending Balances at Dec. 31, 2020	$ 2,341,437	$ 1,646,539	$ (6,357)	$ 712,197		$ (10,942)