STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid In Capital	Retained Earnings	AOCI Attributable to Parent
Beginning Balance (in shares) at Dec. 31, 2016		100
Beginning Balance at Dec. 31, 2016	$ 1,931,600	$ 0	$ 1,446,200	$ 486,700	$ (1,300)
Increase (Decrease) in Stockholder's Equity
Net income	159,200			159,200
Other comprehensive income	100				100
Common dividends declared to parent	(104,600)			(104,600)
Contribution of capital by parent	144,000		144,000
Adoption of ASU No. 2018-02	0			300	(300)
Ending Balance (in shares) at Dec. 31, 2017		100
Ending Balance at Dec. 31, 2017	2,130,300	$ 0	1,590,200	541,600	(1,500)
Increase (Decrease) in Stockholder's Equity
Net income	213,300			213,300
Other comprehensive income	100				100
Common dividends declared to parent	(149,200)			(149,200)
Contribution of capital by parent	$ 342,100		342,100
Ending Balance (in shares) at Dec. 31, 2018	100	100
Ending Balance at Dec. 31, 2018	$ 2,536,600	$ 0	1,932,300	605,700	(1,400)
Increase (Decrease) in Stockholder's Equity
Net income	263,100			263,100
Other comprehensive income	0				0
Common dividends declared to parent	(333,800)			(333,800)
Contribution of capital by parent	$ 418,600		418,600
Ending Balance (in shares) at Dec. 31, 2019	100	100
Ending Balance at Dec. 31, 2019	$ 2,884,500	$ 0	$ 2,350,900	$ 535,000	$ (1,400)