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, 2015	$ 790.4	$ 93.3	$ 394.6	$ 302.7	$ (0.2)
Balance (in shares) at Dec. 31, 2015		933,000
Comprehensive income:
Net income	69.1			69.1
Other comprehensive income	0.1				0.1
Common dividends declared to parent	(48.5)			(48.5)
Contribution of capital by parent	0.8		0.8
Ending Balance at Dec. 31, 2016	811.9	$ 93.3	395.4	323.3	(0.1)
Balance (in shares) at Dec. 31, 2016		933,000
Comprehensive income:
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	933,000
Comprehensive income:
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