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, 2015	$ 3,531,225		$ 3,308,957	$ 249,534	$ (27,266)	$ 3,362,992	$ 859	$ 3,275,105	$ 236,578	$ (149,550)
Balance (in shares) at Dec. 31, 2015		200					85,903,791
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	312,899			312,899		380,581			380,581
Common stock dividend paid	(148,965)			(148,965)		(257,364)			(257,364)
Other comprehensive income (loss)	(6,446)				(6,446)	4,039				4,039
Balance at Dec. 31, 2016	3,688,713		3,308,957	413,468	(33,712)	3,490,248	$ 859	3,275,105	359,795	(145,511)
Balance (in shares) at Dec. 31, 2016		200.0					85,903,791
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	175,194			175,194		320,054			320,054
Common stock dividend paid	(123,307)			(123,307)		(227,783)			(227,783)
Other comprehensive income (loss)	9,430				9,430	18,605				18,605
Balance at Dec. 31, 2017	$ 3,750,030		3,308,957	465,355	(24,282)	$ 3,601,124	$ 859	3,275,105	452,066	(126,906)
Balance (in shares) at Dec. 31, 2017	200	200.0				85,903,791	85,903,791
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 235,622			235,622		$ 317,162			317,162
Common stock dividend paid	(77,204)			(77,204)		(173,716)			(173,716)
Other comprehensive income (loss)	(52,920)				(52,920)	(63,978)				(63,978)
Balance at Dec. 31, 2018	$ 3,860,758		$ 3,308,957	629,003	$ (77,202)	$ 3,707,924	$ 859	$ 3,275,105	622,844	$ (190,884)
Balance (in shares) at Dec. 31, 2018	200	200.0				85,903,791	85,903,791
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	$ 5,200					$ 27,332			$ 27,332
Other Comprehensive Income (Loss) Reclassification of Stranded Taxes to RE for Pension Plans	$ 5,230			$ 5,230		$ 26,200