CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Subsidiaries [Member]	Subsidiaries [Member] Common Stock	Subsidiaries [Member] Additional Paid-in Capital	Subsidiaries [Member] Retained Earnings	Subsidiaries [Member] Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2020	$ 4,139,882		$ 3,313,532	$ 912,787	$ (86,437)	$ 4,181,409	$ 859	$ 3,485,105	$ 876,401	$ (180,956)
Balance (in shares) at Dec. 31, 2020		200					85,903,791
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	260,849			260,849		336,063			336,063
Dividends, Common Stock	(106,420)			(106,420)		(229,857)			(229,857)
Other comprehensive income (loss)	59,005				59,005	67,815				67,815
Investment from parent	210,000		210,000			0
Balance at Dec. 31, 2021	4,563,316		3,523,532	1,067,216	(27,432)	4,355,430	$ 859	3,485,105	982,607	(113,141)
Balance (in shares) at Dec. 31, 2021		200					85,903,791
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	414,345			414,345		490,952			490,952
Dividends, Common Stock	(16,230)			(16,230)		(35,396)			(35,396)
Other comprehensive income (loss)	2,658				2,658	10,097				10,097
Investment from parent	0					50,000		50,000
Balance at Dec. 31, 2022	$ 4,964,089		3,523,532	1,465,331	(24,774)	$ 4,871,083	$ 859	3,535,105	1,438,163	(103,044)
Balance (in shares) at Dec. 31, 2022	200	200				85,903,791	85,903,791
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 53,740			53,740		$ 131,059			131,059
Dividends, Common Stock	(99,760)			(99,760)		(96,004)			(96,004)
Other comprehensive income (loss)	42,313				42,313	44,650				44,650
Investment from parent	0					100,000		100,000
Balance at Dec. 31, 2023	$ 4,960,382		$ 3,523,532	$ 1,419,311	$ 17,539	$ 5,050,788	$ 859	$ 3,635,105	$ 1,473,218	$ (58,394)
Balance (in shares) at Dec. 31, 2023	200	200				85,903,791	85,903,791