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	444.8			444.8
Total other comprehensive income	0.9				0.9
Dividends declared to parent	(301.1)			(301.1)
Contribution of capital by parent	562.8		562.8
Balance (in shares) at Sep. 30, 2019		100
Ending Balance at Sep. 30, 2019	7,005.6	$ 0.0	4,903.3	2,126.9	(24.6)
Balance (in shares) at Jun. 30, 2019		100
Beginning Balance at Jun. 30, 2019	6,613.1	$ 0.0	4,618.3	2,019.7	(24.9)
Increase (Decrease) in Stockholder's Equity
Net income	204.5			204.5
Total other comprehensive income	0.3				0.3
Dividends declared to parent	(97.3)			(97.3)
Contribution of capital by parent	285.0		285.0
Balance (in shares) at Sep. 30, 2019		100
Ending Balance at Sep. 30, 2019	$ 7,005.6	$ 0.0	4,903.3	2,126.9	(24.6)
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	455.2			455.2
Total other comprehensive income	0.9				0.9
Dividends declared to parent	(694.9)			(694.9)
Contribution of capital by parent	754.0		754.0
Adoption of ASC Topic 326	$ (0.7)			(0.7)
Balance (in shares) at Sep. 30, 2020	100	100
Ending Balance at Sep. 30, 2020	$ 7,510.7	$ 0.0	5,693.4	1,843.0	(25.7)
Balance (in shares) at Jun. 30, 2020		100
Beginning Balance at Jun. 30, 2020	7,420.1	$ 0.0	5,693.7	1,752.4	(26.0)
Increase (Decrease) in Stockholder's Equity
Net income	218.4			218.4
Total other comprehensive income	0.3				0.3
Dividends declared to parent	(127.8)			(127.8)
Contribution of capital by parent	$ (0.3)		(0.3)
Balance (in shares) at Sep. 30, 2020	100	100
Ending Balance at Sep. 30, 2020	$ 7,510.7	$ 0.0	$ 5,693.4	$ 1,843.0	$ (25.7)