CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Millions	Total	Common stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance (in shares) at Dec. 31, 2017		100
Beginning balance at Dec. 31, 2017	$ 5,828.3	$ 0.0	$ 4,032.8	$ 1,822.2	$ (26.7)
Increase (Decrease) in Stockholder's Equity
Net income	463.1			463.1
Other comprehensive income	0.9
Dividends declared to parent	(299.1)			(299.1)
Contribution of capital by parent	245.6		245.6
Balance (in shares) at Sep. 30, 2018		100
Ending balance at Sep. 30, 2018	6,238.8	$ 0.0	4,278.4	1,986.2	(25.8)
Balance (in shares) at Jun. 30, 2018		100
Beginning balance at Jun. 30, 2018	6,129.6	$ 0.0	4,273.1	1,882.6	(26.1)
Increase (Decrease) in Stockholder's Equity
Net income	207.1			207.1
Other comprehensive income	0.3
Dividends declared to parent	(103.5)			(103.5)
Contribution of capital by parent	5.3		5.3
Balance (in shares) at Sep. 30, 2018		100
Ending balance at Sep. 30, 2018	$ 6,238.8	$ 0.0	4,278.4	1,986.2	(25.8)
Balance (in shares) at Dec. 31, 2018	100	100
Beginning balance at Dec. 31, 2018	$ 6,298.2	$ 0.0	4,340.5	1,983.2	(25.5)
Increase (Decrease) in Stockholder's Equity
Net income	444.8			444.8
Other comprehensive income	0.9
Dividends declared to parent	(301.1)			(301.1)
Contribution of capital by parent	$ 562.8		562.8
Balance (in shares) at Sep. 30, 2019	100	100
Ending balance at Sep. 30, 2019	$ 7,005.6	$ 0.0	4,903.3	2,126.9	(24.6)
Balance (in shares) at Jun. 30, 2019		100
Beginning balance at Jun. 30, 2019	6,613.1	$ 0.0	4,618.3	2,019.7	(24.9)
Increase (Decrease) in Stockholder's Equity
Net income	204.5			204.5
Other comprehensive income	0.3
Dividends declared to parent	(97.3)			(97.3)
Contribution of capital by parent	$ 285.0		285.0
Balance (in shares) at Sep. 30, 2019	100	100
Ending balance at Sep. 30, 2019	$ 7,005.6	$ 0.0	$ 4,903.3	$ 2,126.9	$ (24.6)