CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Common Stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Balance (in shares) at Dec. 31, 2018		1,000,000
Beginning balance at Dec. 31, 2018	$ 5,573.1	$ 0.0	$ 3,624.2	$ 1,972.0	$ (23.1)
Increase (Decrease) in Stockholders' Equity
Net income	113.2			113.2
Other comprehensive income	0.2				0.2
Dividends declared on common stock	(94.6)			(94.6)
Contribution of capital by parent	170.0		170.0
Balance (in shares) at Mar. 31, 2019		1,000,000
Ending balance at Mar. 31, 2019	$ 5,761.9	$ 0.0	3,794.2	1,990.6	(22.9)
Balance (in shares) at Dec. 31, 2019	1,000,000	1,000,000
Beginning balance at Dec. 31, 2019	$ 6,081.8	$ 0.0	4,067.9	2,036.4	(22.5)
Increase (Decrease) in Stockholders' Equity
Net income	107.0			107.0
Other comprehensive income	0.2				0.2
Dividends declared on common stock	(99.6)			(99.6)
Contribution of capital by parent	$ 80.0		80.0
Balance (in shares) at Mar. 31, 2020	1,000,000	1,000,000
Ending balance at Mar. 31, 2020	$ 6,168.6	$ 0.0	$ 4,147.9	2,043.0	$ (22.3)
Increase (Decrease) in Stockholders' Equity
Adoption of ASC Topic 326	$ (0.8)			$ (0.8)