Schedule I - Condensed Financial Information Of Registrant (Fair Value) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Apr. 08, 2020
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	$ 19,234	$ 22,945
Investments, Fair Value Disclosure	3,552	3,228
Total financial assets measured at fair value	19,234	22,945
Face Value	3,488,800	3,033,800
Fair Value	3,315,467	2,775,644
Long-term debt	3,003,420	3,000,450
Deferred financing costs, net	23,612
Unamortized discounts	6,768
Interest Rate Swap [Member]
Accumulated Other Income (Loss)			$ 72,300
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative, Notional Amount, Purchase (Sales), Net	400,000
Fixed Rate [Member]
Face Value	3,033,800	3,033,800
Fair Value	2,860,467	2,775,644
Variable Rate [Member]
Face Value	455,000	0
Fair Value	455,000	0
Parent Company [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	17,846	15,400
Investments, Fair Value Disclosure	3,552	3,228
Total financial assets measured at fair value	17,846	15,400
Face Value	880,000	880,000
Fair Value	839,471	816,411
Long-term debt	470,653	873,663
Deferred financing costs, net	4,554	5,912
Unamortized discounts	319	425
Parent Company [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative, Notional Amount, Purchase (Sales), Net	400,000
Parent Company [Member] | Fixed Rate [Member]
Face Value	880,000	880,000
Fair Value	$ 839,471	816,411
Parent Company [Member] | Four Point Two Five Percent Senior Secured Notes
Debt instrument, stated interest rate	4.25%
Long-term debt	$ 475,000	475,000
Subsidiaries [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	1,388	7,545
Total financial assets measured at fair value	1,388	7,545
Face Value	2,608,800	2,153,800
Fair Value	2,475,997	1,959,233
Long-term debt	2,128,293	2,126,787
First mortgage bonds	2,128,293	2,126,787
Deferred financing costs, net	20,200	20,400
Unamortized discounts	6,449	6,651
Subsidiaries [Member] | Fixed Rate [Member]
Face Value	2,153,800	2,153,800
Fair Value	2,020,997	1,959,233
Subsidiaries [Member] | Variable Rate [Member]
Face Value	455,000	0
Fair Value	455,000	0
Unamortized discounts	$ (6,400)	6,700
Subsidiaries [Member] | FMB Twenty - three
Debt instrument, stated interest rate	0.95%
First mortgage bonds	$ 60,000	60,000
Fair Value, Inputs, Level 1 [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	3,552	3,228
Investments, Fair Value Disclosure	3,552	3,228
Total financial assets measured at fair value	3,552	3,228
Fair Value, Inputs, Level 1 [Member] | Parent Company [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	3,552	3,228
Investments, Fair Value Disclosure	3,552	3,228
Total financial assets measured at fair value	3,552	3,228
Fair Value, Inputs, Level 1 [Member] | Subsidiaries [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	0	0
Total financial assets measured at fair value	0	0
Significant Observable Inputs (Level 2) [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	14,294	12,172
Investments, Fair Value Disclosure	0	0
Total financial assets measured at fair value	14,294	12,172
Significant Observable Inputs (Level 2) [Member] | Parent Company [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	14,294	12,172
Investments, Fair Value Disclosure	0	0
Total financial assets measured at fair value	14,294	12,172
Significant Observable Inputs (Level 2) [Member] | Subsidiaries [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	0	0
Total financial assets measured at fair value	0	0
Unobservable Inputs (Level 3) [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	1,388	7,545
Investments, Fair Value Disclosure	0	0
Total financial assets measured at fair value	1,388	7,545
Unobservable Inputs (Level 3) [Member] | Parent Company [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	0	0
Investments, Fair Value Disclosure		0
Total financial assets measured at fair value	0	0
Unobservable Inputs (Level 3) [Member] | Subsidiaries [Member]
Condensed Financial Information Disclosure [Abstract]
Total financial assets measured at fair value	1,388	7,545
Total financial assets measured at fair value	1,388	7,545
Money Market Funds [Member]
Investments, Fair Value Disclosure	127	5
Money Market Funds [Member] | Parent Company [Member]
Investments, Fair Value Disclosure	127	5
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Parent Company [Member]
Investments, Fair Value Disclosure	127	5
Money Market Funds [Member] | Significant Observable Inputs (Level 2) [Member] | Parent Company [Member]
Investments, Fair Value Disclosure	0	0
Money Market Funds [Member] | Unobservable Inputs (Level 3) [Member] | Parent Company [Member]
Investments, Fair Value Disclosure	0	0
Mutual Fund [Member]
Investments, Fair Value Disclosure	3,425	3,223
Mutual Fund [Member] | Parent Company [Member]
Investments, Fair Value Disclosure	3,425	3,223
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member] | Parent Company [Member]
Investments, Fair Value Disclosure	3,425	3,223
Mutual Fund [Member] | Significant Observable Inputs (Level 2) [Member] | Parent Company [Member]
Investments, Fair Value Disclosure	0	0
Mutual Fund [Member] | Unobservable Inputs (Level 3) [Member] | Parent Company [Member]
Investments, Fair Value Disclosure	$ 0	$ 0