CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Millions	Total	Common stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2017	$ 876.6	$ 93.3	$ 449.4	$ 334.0	$ (0.1)
Balance (in shares) at Dec. 31, 2017		933,000.0
Increase (Decrease) in Stockholder's Equity
Net income	46.6			46.6
Other comprehensive income	0.1				0.1
Common dividends declared to parent	(32.5)			(32.5)
Contribution of capital by parent	3.5		3.5
Ending Balance at Jun. 30, 2018	894.3	$ 93.3	452.9	348.1	0.0
Balance (in shares) at Jun. 30, 2018		933,000.0
Beginning Balance at Mar. 31, 2018	892.0	$ 93.3	449.4	349.4	(0.1)
Balance (in shares) at Mar. 31, 2018		933,000.0
Increase (Decrease) in Stockholder's Equity
Net income	15.2			15.2
Other comprehensive income	0.1				0.1
Common dividends declared to parent	(16.5)			(16.5)
Contribution of capital by parent	3.5		3.5
Ending Balance at Jun. 30, 2018	894.3	$ 93.3	452.9	348.1	0.0
Balance (in shares) at Jun. 30, 2018		933,000.0
Beginning Balance at Dec. 31, 2018	$ 942.4	$ 93.3	510.1	339.0	0.0
Balance (in shares) at Dec. 31, 2018	933,000	933,000.0
Increase (Decrease) in Stockholder's Equity
Net income	$ 32.6			32.6
Other comprehensive income	0.0				0.0
Common dividends declared to parent	(40.3)			(40.3)
Contribution of capital by parent	3.5		3.5
Ending Balance at Jun. 30, 2019	$ 938.2	$ 93.3	513.6	331.3	0.0
Balance (in shares) at Jun. 30, 2019	933,000	933,000.0
Beginning Balance at Mar. 31, 2019	$ 942.0	$ 93.3	510.1	338.6	0.0
Balance (in shares) at Mar. 31, 2019		933,000.0
Increase (Decrease) in Stockholder's Equity
Net income	8.6			8.6
Other comprehensive income	0.0				0.0
Common dividends declared to parent	(15.9)			(15.9)
Contribution of capital by parent	3.5		3.5
Ending Balance at Jun. 30, 2019	$ 938.2	$ 93.3	$ 513.6	$ 331.3	$ 0.0
Balance (in shares) at Jun. 30, 2019	933,000	933,000.0