CONDENSED CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Millions	Total	Total Common Shareholders' Equity	Common stock	Additional paid In capital	Retained earnings
Balance at Dec. 31, 2017		$ 1,617.3	$ 95.6	$ 996.1	$ 525.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		49.8	0.0	0.0	49.8
Payment of dividends to parent		(30.0)	0.0	0.0	(30.0)
Balance at Mar. 31, 2018		1,637.1	95.6	996.1	545.4
Balance at Dec. 31, 2017		1,617.3	95.6	996.1	525.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 92.5
Equity contribution from parent	(70.0)
Balance at Jun. 30, 2018		1,719.5	95.6	1,065.8	558.1
Balance at Mar. 31, 2018		1,637.1	95.6	996.1	545.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	42.7	42.7	0.0	0.0	42.7
Equity contribution from parent		70.0	0.0	70.0	0.0
Payment of dividends to parent		(30.0)	0.0	0.0	(30.0)
Stock-based compensation and other		(0.3)	0.0	(0.3)	0.0
Balance at Jun. 30, 2018		1,719.5	95.6	1,065.8	558.1
Balance at Dec. 31, 2018		1,769.9	95.6	1,115.9	558.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		40.9	0.0	0.0	40.9
Equity contribution from parent		105.0	0.0	105.0	0.0
Payment of dividends to parent		(30.0)	0.0	0.0	(30.0)
Balance at Mar. 31, 2019		1,885.8	95.6	1,220.9	569.3
Balance at Dec. 31, 2018		1,769.9	95.6	1,115.9	558.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	82.4
Equity contribution from parent	(105.0)
Balance at Jun. 30, 2019		1,897.4	95.6	1,221.0	580.8
Balance at Mar. 31, 2019		1,885.8	95.6	1,220.9	569.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 41.5	41.5	0.0	0.0	41.5
Payment of dividends to parent		(30.0)	0.0	0.0	(30.0)
Stock-based compensation and other		0.1	0.0	0.1	0.0
Balance at Jun. 30, 2019		$ 1,897.4	$ 95.6	$ 1,221.0	$ 580.8