CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Millions	Total	Common stock	Additional Paid In Capital	Retained Earnings
Balance (in shares) at Dec. 31, 2018		933,000.0
Beginning Balance at Dec. 31, 2018	$ 942.4	$ 93.3	$ 510.1	$ 339.0
Increase (Decrease) in Stockholder's Equity
Net income	24.0
Common dividends declared to parent	(24.4)			(24.4)
Balance (in shares) at Mar. 31, 2019		933,000.0
Ending Balance at Mar. 31, 2019	$ 942.0	$ 93.3	510.1	338.6
Balance (in shares) at Dec. 31, 2019	933,000	933,000.0
Beginning Balance at Dec. 31, 2019	$ 966.6	$ 93.3	537.3	336.0
Increase (Decrease) in Stockholder's Equity
Net income	33.6
Common dividends declared to parent	(16.9)			(16.9)
Contribution of capital by parent	10.0		10.0
Credit Losses, Topic 326 (ASC Topic 326)	$ (0.1)			(0.1)
Balance (in shares) at Mar. 31, 2020	933,000	933,000.0
Ending Balance at Mar. 31, 2020	$ 993.2	$ 93.3	$ 547.3	$ 352.6