CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDER'S EQUITY - USD ($)	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, 2016	$ 3,688,713,000	$ 0	$ 3,308,957,000	$ 413,468,000	$ (33,712,000)	$ 3,490,248,000	$ 859,000	$ 3,275,105,000	$ 359,795,000	$ (145,511,000)
Balance (in shares) at Dec. 31, 2016		200					85,903,791
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	175,194,000			175,194,000		320,054,000			320,054,000
Dividends, Common Stock	(123,307,000)			(123,307,000)		(227,783,000)			(227,783,000)
Other comprehensive income (loss)	9,430,000				9,430,000	18,605,000				18,605,000
Proceeds from Contributions from Parent						0
Reclassification of stranded taxes to retained earnings due to tax reform	0					0
Balance at Dec. 31, 2017	3,750,030,000		3,308,957,000	465,355,000	(24,282,000)	3,601,124,000	$ 859,000	3,275,105,000	452,066,000	(126,906,000)
Balance (in shares) at Dec. 31, 2017		200					85,903,791
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	235,622,000			235,622,000		317,162,000			317,162,000
Dividends, Common Stock	(77,204,000)			(77,204,000)		(173,716,000)			(173,716,000)
Other comprehensive income (loss)	(52,920,000)				(52,920,000)	(63,978,000)				(63,978,000)
Proceeds from Contributions from Parent						0
Reclassification of stranded taxes to retained earnings due to tax reform	(5,230,000)					27,333,000			27,332,000
Other Comprehensive Income (Loss) Reclassification of Stranded Taxes to RE for Pension Plans	5,230,000			5,230,000
Balance at Dec. 31, 2018	$ 3,860,758,000		3,308,957,000	629,003,000	(77,202,000)	$ 3,707,924,000	$ 859,000	3,275,105,000	622,844,000	(190,884,000)
Balance (in shares) at Dec. 31, 2018	200	200				85,903,791	85,903,791
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 210,708,000			210,708,000		$ 292,924,000			292,924,000
Dividends, Common Stock	(64,220,000)			(64,220,000)		(164,575,000)			(164,575,000)
Other comprehensive income (loss)	(6,947,000)				(6,947,000)	2,407,000				2,407,000
Proceeds from Contributions from Parent						210,000,000		(210,000,000)
Reclassification of stranded taxes to retained earnings due to tax reform	0					0
Balance at Dec. 31, 2019	$ 4,000,299,000		$ 3,308,957,000	$ 775,491,000	$ (84,149,000)	$ 4,048,680,000	$ 859,000	$ 3,485,105,000	$ 751,193,000	$ (188,477,000)
Balance (in shares) at Dec. 31, 2019	200	200				85,903,791	85,903,791