CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Common Stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning balance (in shares) at Dec. 31, 2018		100
Beginning balance at Dec. 31, 2018	$ 2,536.6	$ 0.0	$ 1,932.3	$ 605.7	$ (1.4)
Increase (Decrease) in Stockholders' Equity
Net Income (Loss) Attributable to Parent	218.0			218.0
Other comprehensive income	0.1				0.1
Dividends declared to parent	(255.5)			(255.5)
Contributions of capital by parent	393.0		393.0
Ending balance (in shares) at Sep. 30, 2019		100
Ending balance at Sep. 30, 2019	2,892.2	$ 0.0	2,325.3	568.2	(1.3)
Beginning balance (in shares) at Jun. 30, 2019		100
Beginning balance at Jun. 30, 2019	2,883.7	$ 0.0	2,307.3	577.7	(1.3)
Increase (Decrease) in Stockholders' Equity
Net Income (Loss) Attributable to Parent	105.1			105.1
Other comprehensive income	0.0
Dividends declared to parent	(114.6)			(114.6)
Contributions of capital by parent	18.0		18.0
Ending balance (in shares) at Sep. 30, 2019		100
Ending balance at Sep. 30, 2019	$ 2,892.2	$ 0.0	2,325.3	568.2	(1.3)
Beginning balance (in shares) at Dec. 31, 2019	100	100
Beginning balance at Dec. 31, 2019	$ 2,884.5	$ 0.0	2,350.9	535.0	(1.4)
Increase (Decrease) in Stockholders' Equity
Net Income (Loss) Attributable to Parent	241.5			241.5
Other comprehensive income	0.1				0.1
Dividends declared to parent	(266.2)			(266.2)
Contributions of capital by parent	$ 436.0		436.0
Ending balance (in shares) at Sep. 30, 2020	100	100
Ending balance at Sep. 30, 2020	$ 3,295.8	$ 0.0	2,786.9	510.2	(1.3)
Increase (Decrease) in Stockholders' Equity
Credit Losses, Topic 326 (ASC Topic 326)	(0.1)			(0.1)
Beginning balance (in shares) at Jun. 30, 2020		100
Beginning balance at Jun. 30, 2020	3,304.7	$ 0.0	2,786.9	519.1	(1.3)
Increase (Decrease) in Stockholders' Equity
Net Income (Loss) Attributable to Parent	127.1			127.1
Other comprehensive income	0.0
Dividends declared to parent	$ (136.0)			(136.0)
Ending balance (in shares) at Sep. 30, 2020	100	100
Ending balance at Sep. 30, 2020	$ 3,295.8	$ 0.0	$ 2,786.9	$ 510.2	$ (1.3)