CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Common Stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning balance (in shares) at Dec. 31, 2017		100
Beginning balance at Dec. 31, 2017	$ 2,130.3	$ 0.0	$ 1,590.2	$ 541.6	$ (1.5)
Increase (Decrease) in Stockholders' Equity
Other comprehensive income	0.1				0.1
Ending balance (in shares) at Jun. 30, 2018		100
Ending balance at Jun. 30, 2018	2,159.1	$ 0.0	1,591.4	569.2	(1.5)
Beginning balance (in shares) at Dec. 31, 2017		100
Beginning balance at Dec. 31, 2017	2,130.3	$ 0.0	1,590.2	541.6	(1.5)
Increase (Decrease) in Stockholders' Equity
Net income	173.1			173.1
Other comprehensive income	0.1
Dividends declared to parent	(104.0)			(104.0)
Contributions of capital by parent	181.2		181.2
Ending balance (in shares) at Sep. 30, 2018		100
Ending balance at Sep. 30, 2018	2,380.7	$ 0.0	1,771.4	610.7	(1.4)
Beginning balance (in shares) at Jun. 30, 2018		100
Beginning balance at Jun. 30, 2018	2,159.1	$ 0.0	1,591.4	569.2	(1.5)
Increase (Decrease) in Stockholders' Equity
Net income	81.5			81.5
Other comprehensive income	0.1
Dividends declared to parent	(40.0)			(40.0)
Contributions of capital by parent	180.0		180.0
Ending balance (in shares) at Sep. 30, 2018		100
Ending balance at Sep. 30, 2018	$ 2,380.7	$ 0.0	1,771.4	610.7	(1.4)
Beginning balance (in shares) at Dec. 31, 2018	100	100
Beginning balance at Dec. 31, 2018	$ 2,536.6	$ 0.0	1,932.3	605.7	(1.4)
Increase (Decrease) in Stockholders' Equity
Net income	218.0			218.0
Other comprehensive income	0.1				0.1
Dividends declared to parent	(255.5)			(255.5)
Contributions of capital by parent	$ 393.0		393.0
Ending balance (in shares) at Sep. 30, 2019	100	100
Ending balance at Sep. 30, 2019	$ 2,892.2	$ 0.0	2,325.3	568.2	(1.3)
Beginning balance (in shares) at Jun. 30, 2019		100
Beginning balance at Jun. 30, 2019	2,883.7	$ 0.0	2,307.3	577.7	(1.3)
Increase (Decrease) in Stockholders' Equity
Net income	105.1			105.1
Other comprehensive income	0.0
Dividends declared to parent	(114.6)			(114.6)
Contributions of capital by parent	$ 18.0		18.0
Ending balance (in shares) at Sep. 30, 2019	100	100
Ending balance at Sep. 30, 2019	$ 2,892.2	$ 0.0	$ 2,325.3	$ 568.2	$ (1.3)