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
Net income	91.6			91.6
Other comprehensive income	0.0
Dividends declared to parent	(64.0)			(64.0)
Contributions of capital by parent	1.2		1.2
Ending balance at Jun. 30, 2018	2,159.1	$ 0.0	1,591.4	569.2	(1.5)
Ending balance (in shares) at Jun. 30, 2018		100
Beginning balance (in shares) at Mar. 31, 2018		100
Beginning balance at Mar. 31, 2018	2,130.1	$ 0.0	1,590.2	541.4	(1.5)
Increase (Decrease) in Stockholders' Equity
Net income	58.5			58.5
Other comprehensive income	0.0
Dividends declared to parent	(30.7)			(30.7)
Contributions of capital by parent	1.2		1.2
Ending balance at Jun. 30, 2018	$ 2,159.1	$ 0.0	1,591.4	569.2	(1.5)
Ending balance (in shares) at Jun. 30, 2018		100
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	112.9			112.9
Other comprehensive income	0.1				0.1
Dividends declared to parent	(140.9)			(140.9)
Contributions of capital by parent	375.0		375.0
Ending balance at Jun. 30, 2019	$ 2,883.7	$ 0.0	2,307.3	577.7	(1.3)
Ending balance (in shares) at Jun. 30, 2019	100	100
Beginning balance (in shares) at Mar. 31, 2019		100
Beginning balance at Mar. 31, 2019	$ 2,533.2	$ 0.0	1,932.3	602.3	(1.4)
Increase (Decrease) in Stockholders' Equity
Net income	58.8			58.8
Other comprehensive income	0.1				0.1
Dividends declared to parent	(83.4)			(83.4)
Contributions of capital by parent	375.0		375.0
Ending balance at Jun. 30, 2019	$ 2,883.7	$ 0.0	$ 2,307.3	$ 577.7	$ (1.3)
Ending balance (in shares) at Jun. 30, 2019	100	100