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, 2017		933,000.0
Beginning Balance at Dec. 31, 2017	$ 876.6	$ 93.3	$ 449.4	$ 334.0	$ (0.1)
Increase (Decrease) in Stockholder's Equity
Net income	77.6
Other comprehensive income	0.1				0.1
Common dividends declared to parent	(50.7)			(50.7)
Contribution of capital by parent	3.9		3.9
Balance (in shares) at Sep. 30, 2018		933,000.0
Ending Balance at Sep. 30, 2018	907.5	$ 93.3	453.3	360.9	0.0
Balance (in shares) at Jun. 30, 2018		933,000.0
Beginning Balance at Jun. 30, 2018	894.3	$ 93.3	452.9	348.1	0.0
Increase (Decrease) in Stockholder's Equity
Net income	31.0
Other comprehensive income	0.0
Common dividends declared to parent	(18.2)			(18.2)
Contribution of capital by parent	0.4		0.4
Balance (in shares) at Sep. 30, 2018		933,000.0
Ending Balance at Sep. 30, 2018	$ 907.5	$ 93.3	453.3	360.9	0.0
Balance (in shares) at Dec. 31, 2018	933,000	933,000.0
Beginning Balance at Dec. 31, 2018	$ 942.4	$ 93.3	510.1	339.0	0.0
Increase (Decrease) in Stockholder's Equity
Net income	61.9
Other comprehensive income	0.0
Common dividends declared to parent	(67.1)			(67.1)
Contribution of capital by parent	$ 3.5		3.5
Balance (in shares) at Sep. 30, 2019	933,000	933,000.0
Ending Balance at Sep. 30, 2019	$ 940.7	$ 93.3	513.6	333.8	0.0
Balance (in shares) at Jun. 30, 2019		933,000.0
Beginning Balance at Jun. 30, 2019	938.2	$ 93.3	513.6	331.3	0.0
Increase (Decrease) in Stockholder's Equity
Net income	29.4
Other comprehensive income	0.0
Common dividends declared to parent	$ (26.9)			(26.9)
Balance (in shares) at Sep. 30, 2019	933,000	933,000.0
Ending Balance at Sep. 30, 2019	$ 940.7	$ 93.3	$ 513.6	$ 333.8	$ 0.0