CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Millions	Total	Common stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance (in shares) at Dec. 31, 2018		100
Beginning Balance at Dec. 31, 2018	$ 6,298.2	$ 0.0	$ 4,340.5	$ 1,983.2	$ (25.5)
Increase (Decrease) in Stockholder's Equity
Net income	240.3			240.3
Other comprehensive income	0.6				0.6
Common dividends declared to parent	(203.8)			(203.8)
Contribution of capital by parent	277.8		277.8
Balance (in shares) at Jun. 30, 2019		100
Ending Balance at Jun. 30, 2019	6,613.1	$ 0.0	4,618.3	2,019.7	(24.9)
Balance (in shares) at Mar. 31, 2019		100
Beginning Balance at Mar. 31, 2019	6,388.4	$ 0.0	4,390.5	2,023.1	(25.2)
Increase (Decrease) in Stockholder's Equity
Net income	101.5			101.5
Other comprehensive income	0.3				0.3
Common dividends declared to parent	(104.9)			(104.9)
Contribution of capital by parent	227.8		227.8
Balance (in shares) at Jun. 30, 2019		100
Ending Balance at Jun. 30, 2019	$ 6,613.1	$ 0.0	4,618.3	2,019.7	(24.9)
Balance (in shares) at Dec. 31, 2019	100	100
Beginning Balance at Dec. 31, 2019	$ 6,996.2	$ 0.0	4,939.4	2,083.4	(26.6)
Increase (Decrease) in Stockholder's Equity
Net income	236.8			236.8
Other comprehensive income	0.6				0.6
Common dividends declared to parent	(567.1)			(567.1)
Contribution of capital by parent	754.3		754.3
Credit Losses, Topic 326 (ASC Topic 326)	$ (0.7)			(0.7)
Balance (in shares) at Jun. 30, 2020	100	100
Ending Balance at Jun. 30, 2020	$ 7,420.1	$ 0.0	5,693.7	1,752.4	(26.0)
Balance (in shares) at Mar. 31, 2020		100
Beginning Balance at Mar. 31, 2020	7,072.0	$ 0.0	4,989.4	2,108.9	(26.3)
Increase (Decrease) in Stockholder's Equity
Net income	108.1			108.1
Other comprehensive income	0.3				0.3
Common dividends declared to parent	(464.6)			(464.6)
Contribution of capital by parent	$ 704.3		704.3
Balance (in shares) at Jun. 30, 2020	100	100
Ending Balance at Jun. 30, 2020	$ 7,420.1	$ 0.0	$ 5,693.7	$ 1,752.4	$ (26.0)