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	54.1			54.1
Other comprehensive income	0.0
Dividends declared to parent	(57.5)			(57.5)
Ending balance (in shares) at Mar. 31, 2019		100
Ending balance at Mar. 31, 2019	$ 2,533.2	$ 0.0	1,932.3	602.3	(1.4)
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	42.7			42.7
Other comprehensive income	0.0
Dividends declared to parent	(75.6)			(75.6)
Contributions of capital by parent	$ 32.0		32.0
Ending balance (in shares) at Mar. 31, 2020	100	100
Ending balance at Mar. 31, 2020	$ 2,883.5	$ 0.0	$ 2,382.9	502.0	$ (1.4)
Increase (Decrease) in Stockholders' Equity
Credit Losses, Topic 326 (ASC Topic 326)	$ (0.1)			$ (0.1)