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	209.1			209.1
Other comprehensive income	0.4				0.4
Dividends declared on common stock	(189.9)			(189.9)
Contribution of capital by parent	174.8		174.8
Balance (in shares) at Jun. 30, 2019		1,000,000
Ending balance at Jun. 30, 2019	5,767.5	$ 0.0	3,799.0	1,991.2	(22.7)
Balance (in shares) at Mar. 31, 2019		1,000,000
Beginning balance at Mar. 31, 2019	5,761.9	$ 0.0	3,794.2	1,990.6	(22.9)
Increase (Decrease) in Stockholders' Equity
Net income	95.9			95.9
Other comprehensive income	0.2				0.2
Dividends declared on common stock	(95.3)			(95.3)
Contribution of capital by parent	4.8		4.8
Balance (in shares) at Jun. 30, 2019		1,000,000
Ending balance at Jun. 30, 2019	$ 5,767.5	$ 0.0	3,799.0	1,991.2	(22.7)
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	224.4			224.4
Other comprehensive income	0.5				0.5
Dividends declared on common stock	(205.0)			(205.0)
Contribution of capital by parent	$ 188.9		188.9
Balance (in shares) at Jun. 30, 2020	1,000,000	1,000,000
Ending balance at Jun. 30, 2020	$ 6,289.8	$ 0.0	4,256.8	2,055.0	(22.0)
Increase (Decrease) in Stockholders' Equity
Adoption of ASC Topic 326				(0.8)
Balance (in shares) at Mar. 31, 2020		1,000,000
Beginning balance at Mar. 31, 2020	6,168.6	$ 0.0	4,147.9	2,043.0	(22.3)
Increase (Decrease) in Stockholders' Equity
Net income	117.4			117.4
Other comprehensive income	0.3				0.3
Dividends declared on common stock	(105.4)			(105.4)
Contribution of capital by parent	$ 108.9		108.9
Balance (in shares) at Jun. 30, 2020	1,000,000	1,000,000
Ending balance at Jun. 30, 2020	$ 6,289.8	$ 0.0	$ 4,256.8	$ 2,055.0	$ (22.0)