CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Common Stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Common Stock, Dividends, Per Share, Declared	$ 0
Beginning balance at Dec. 31, 2017	$ 2,130.3	$ 0.0	$ 1,590.2	$ 541.6	$ (1.5)
Balance (in shares) at Dec. 31, 2017		100
Increase (Decrease) in Stockholders' Equity
Net income	33.1			33.1
Other comprehensive income	0.1				0.1
Dividends declared on common stock	(33.3)			(33.3)
Ending balance at Mar. 31, 2018	$ 2,130.2	$ 0.0	1,590.2	541.4	(1.4)
Balance (in shares) at Mar. 31, 2018		100
Common Stock, Dividends, Per Share, Declared	$ 0
Beginning balance at Dec. 31, 2018	$ 2,536.6	$ 0.0	1,932.3	605.7	(1.4)
Balance (in shares) at Dec. 31, 2018	100	100
Increase (Decrease) in Stockholders' Equity
Net income	$ 54.1			54.1
Other comprehensive income	0.0
Dividends declared on common stock	(57.5)			(57.5)
Ending balance at Mar. 31, 2019	$ 2,533.2	$ 0.0	$ 1,932.3	$ 602.3	$ (1.4)
Balance (in shares) at Mar. 31, 2019	100	100