CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Millions	Total	Common stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2016	$ 811.9	$ 93.3	$ 395.4	$ 323.3	$ (0.1)
Balance (in shares) at Dec. 31, 2016		933,000
Increase (Decrease) in Stockholder's Equity
Net income	79.4			79.4
Other comprehensive income	0.0				0.0
Common dividends declared to parent	(68.7)			(68.7)
Contribution of capital by parent	54.0		54.0
Ending Balance at Dec. 31, 2017	876.6	$ 93.3	449.4	334.0	(0.1)
Balance (in shares) at Dec. 31, 2017		933,000
Increase (Decrease) in Stockholder's Equity
Net income	98.0			98.0
Other comprehensive income	0.1				0.1
Common dividends declared to parent	(93.0)			(93.0)
Contribution of capital by parent	60.7		60.7
Ending Balance at Dec. 31, 2018	$ 942.4	$ 93.3	510.1	339.0	0.0
Balance (in shares) at Dec. 31, 2018	933,000	933,000
Increase (Decrease) in Stockholder's Equity
Net income	$ 78.9			78.9
Other comprehensive income	0.0				0.0
Common dividends declared to parent	(81.9)			(81.9)
Contribution of capital by parent	27.2		27.2
Ending Balance at Dec. 31, 2019	$ 966.6	$ 93.3	$ 537.3	$ 336.0	$ 0.0
Balance (in shares) at Dec. 31, 2019	933,000	933,000