CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($)	Total	Common stock	Additional Paid In Capital	Retained Earnings
Balance (in shares) at Dec. 31, 2018		933,000.0
Beginning Balance at Dec. 31, 2018	$ 942,400,000	$ 93,300,000	$ 510,100,000	$ 339,000,000.0
Increase (Decrease) in Stockholder's Equity
Net Income (Loss) Attributable to Parent	61,900,000
Common dividends declared to parent	(67,100,000)			(67,100,000)
Contribution of capital by parent	3,500,000		3,500,000
Balance (in shares) at Sep. 30, 2019		933,000.0
Ending Balance at Sep. 30, 2019	940,700,000	$ 93,300,000	513,600,000	333,800,000
Balance (in shares) at Jun. 30, 2019		933,000.0
Beginning Balance at Jun. 30, 2019	938,200,000	$ 93,300,000	513,600,000	331,300,000
Increase (Decrease) in Stockholder's Equity
Net Income (Loss) Attributable to Parent	29,400,000
Common dividends declared to parent	(26,900,000)			(26,900,000)
Balance (in shares) at Sep. 30, 2019		933,000.0
Ending Balance at Sep. 30, 2019	$ 940,700,000	$ 93,300,000	513,600,000	333,800,000
Balance (in shares) at Dec. 31, 2019	933,000	933,000.0
Beginning Balance at Dec. 31, 2019	$ 966,600,000	$ 93,300,000	537,300,000	336,000,000.0
Increase (Decrease) in Stockholder's Equity
Net Income (Loss) Attributable to Parent	85,200,000
Common dividends declared to parent	(55,200,000)			(55,200,000)
Contribution of capital by parent	40,500,000		40,500,000
Credit Losses, Topic 326 (ASC Topic 326)	$ (100,000)			(100,000)
Balance (in shares) at Sep. 30, 2020	933,000	933,000.0
Ending Balance at Sep. 30, 2020	$ 1,037,000,000.0	$ 93,300,000	577,800,000	365,900,000
Balance (in shares) at Jun. 30, 2020		933,000.0
Beginning Balance at Jun. 30, 2020	1,015,500,000	$ 93,300,000	577,700,000	344,500,000
Increase (Decrease) in Stockholder's Equity
Net Income (Loss) Attributable to Parent	39,500,000
Common dividends declared to parent	(18,100,000)			(18,100,000)
Contribution of capital by parent	$ 100,000		100,000
Balance (in shares) at Sep. 30, 2020	933,000	933,000.0
Ending Balance at Sep. 30, 2020	$ 1,037,000,000.0	$ 93,300,000	$ 577,800,000	$ 365,900,000