CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDER’S EQUITY Statement - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Subsidiaries [Member]	Subsidiaries [Member] Common Stock [Member]	Subsidiaries [Member] Additional Paid-in Capital [Member]	Subsidiaries [Member] Retained Earnings [Member]	Subsidiaries [Member] AOCI Attributable to Parent [Member]
Beginning Balance (in shares) at Dec. 31, 2019		200					85,903,791
Beginning Balance at Dec. 31, 2019	$ 4,000,299	$ 0	$ 3,308,957	$ 775,491	$ (84,149)	$ 4,048,680	$ 859	$ 3,485,105	$ 751,193	$ (188,477)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss) Attributable to Parent	94,936			94,936		111,321			111,321
Dividends, Common Stock	(22,645)			(22,645)		(53,794)			(53,794)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	5,170				5,170	7,806				7,806
Ending Balance (in shares) at Mar. 31, 2020		200					85,903,791
Ending Balance at Mar. 31, 2020	4,077,760	$ 0	3,308,957	847,782	(78,979)	4,114,013	$ 859	3,485,105	808,720	(180,671)
Beginning Balance (in shares) at Dec. 31, 2019		200					85,903,791
Beginning Balance at Dec. 31, 2019	4,000,299	$ 0	3,308,957	775,491	(84,149)	4,048,680	$ 859	3,485,105	751,193	(188,477)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss) Attributable to Parent	81,699					159,420
Proceeds from Contributions from Parent	0
Ending Balance (in shares) at Sep. 30, 2020		200					85,903,791
Ending Balance at Sep. 30, 2020	4,044,476	$ 0	3,308,957	811,778	(76,259)	4,097,603	$ 859	3,485,105	785,062	(173,423)
Beginning Balance (in shares) at Mar. 31, 2020		200					85,903,791
Beginning Balance at Mar. 31, 2020	4,077,760	$ 0	3,308,957	847,782	(78,979)	4,114,013	$ 859	3,485,105	808,720	(180,671)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss) Attributable to Parent	(23,233)			(23,233)		15,037			15,037
Dividends, Common Stock	(22,419)			(22,419)		(46,015)			(46,015)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	1,630				1,630	3,956				3,956
Ending Balance (in shares) at Jun. 30, 2020		200					85,903,791
Ending Balance at Jun. 30, 2020	4,033,738	$ 0	3,308,957	802,130	(77,349)	4,086,991	$ 859	3,485,105	777,742	(176,715)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss) Attributable to Parent	9,996			9,996		33,062			33,062
Dividends, Common Stock	(348)			(348)		(25,742)			(25,742)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	1,090				1,090	3,292				3,292
Ending Balance (in shares) at Sep. 30, 2020		200					85,903,791
Ending Balance at Sep. 30, 2020	$ 4,044,476	$ 0	3,308,957	811,778	(76,259)	$ 4,097,603	$ 859	3,485,105	785,062	(173,423)
Beginning Balance (in shares) at Dec. 31, 2020	200	200				85,903,791	85,903,791
Beginning Balance at Dec. 31, 2020	$ 4,139,882	$ 0	3,313,532	912,787	(86,437)	$ 4,181,409	$ 859	3,485,105	876,401	(180,956)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss) Attributable to Parent	188,993			188,993		199,470			199,470
Dividends, Common Stock	(22,939)			(22,939)		(52,053)			(52,053)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	2,385				2,385	4,554				4,554
Ending Balance (in shares) at Mar. 31, 2021		200					85,903,791
Ending Balance at Mar. 31, 2021	$ 4,308,321	$ 0	3,313,532	1,078,841	(84,052)	$ 4,333,380	$ 859	3,485,105	1,023,818	(176,402)
Beginning Balance (in shares) at Dec. 31, 2020	200	200				85,903,791	85,903,791
Beginning Balance at Dec. 31, 2020	$ 4,139,882	$ 0	3,313,532	912,787	(86,437)	$ 4,181,409	$ 859	3,485,105	876,401	(180,956)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss) Attributable to Parent	293,804					$ 351,959
Proceeds from Contributions from Parent	$ 210,000
Ending Balance (in shares) at Sep. 30, 2021	200	200				85,903,791	85,903,791
Ending Balance at Sep. 30, 2021	$ 4,581,998	$ 0	3,523,532	1,138,524	(80,058)	$ 4,375,486	$ 859	3,485,105	1,057,515	(167,993)
Beginning Balance (in shares) at Mar. 31, 2021		200					85,903,791
Beginning Balance at Mar. 31, 2021	4,308,321	$ 0	3,313,532	1,078,841	(84,052)	4,333,380	$ 859	3,485,105	1,023,818	(176,402)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss) Attributable to Parent	55,242			55,242		78,364			78,364
Dividends, Common Stock	(23,214)			(23,214)		(44,622)			(44,622)
Proceeds from Contributions from Parent	210,000		210,000
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	2,386				2,386	4,555				4,555
Ending Balance (in shares) at Jun. 30, 2021		200					85,903,791
Ending Balance at Jun. 30, 2021	4,552,735	$ 0	3,523,532	1,110,869	(81,666)	4,371,677	$ 859	3,485,105	1,057,560	(171,847)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss) Attributable to Parent	49,569			49,569		74,125			74,125
Dividends, Common Stock	(21,914)			(21,914)		(74,170)			(74,170)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ 1,608				1,608	$ 3,854				3,854
Ending Balance (in shares) at Sep. 30, 2021	200	200				85,903,791	85,903,791
Ending Balance at Sep. 30, 2021	$ 4,581,998	$ 0	$ 3,523,532	$ 1,138,524	$ (80,058)	$ 4,375,486	$ 859	$ 3,485,105	$ 1,057,515	$ (167,993)