CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Millions	Total	Common stock	Additional Paid In Capital	Retained Earnings	AOCI Attributable to Parent
Beginning balance at Dec. 31, 2016	$ 5,355.6	$ 0.0	$ 3,435.1	$ 1,941.3	$ (20.8)
Balance (in shares) at Dec. 31, 2016		1,000,000
Increase (Decrease) in Stockholder's Equity
Net income	490.1			490.1
Other comprehensive income (loss)	0.6				0.6
Common dividends declared to parent	(515.8)			(515.8)
Contribution of capital by parent	145.1		145.1
Adoption of ASU No. 2018-02 (a)				4.3	(4.3)
Ending balance at Dec. 31, 2017	5,475.6	$ 0.0	3,580.2	1,919.9	(24.5)
Balance (in shares) at Dec. 31, 2017		1,000,000
Increase (Decrease) in Stockholder's Equity
Net income	492.3			492.3
Other comprehensive income (loss)	1.4				1.4
Common dividends declared to parent	(440.2)			(440.2)
Contribution of capital by parent	44.0		44.0
Ending balance at Dec. 31, 2018	$ 5,573.1	$ 0.0	3,624.2	1,972.0	(23.1)
Balance (in shares) at Dec. 31, 2018	1,000,000	1,000,000
Increase (Decrease) in Stockholder's Equity
Net income	$ 542.6			542.6
Other comprehensive income (loss)	0.6				0.6
Common dividends declared to parent	(478.2)			(478.2)
Contribution of capital by parent	443.7		443.7
Ending balance at Dec. 31, 2019	$ 6,081.8	$ 0.0	$ 4,067.9	$ 2,036.4	$ (22.5)
Balance (in shares) at Dec. 31, 2019	1,000,000	1,000,000