CONDENSED CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Millions	Total	Total common shareholder's equity	Common stock	Additional paid in capital	Retained earnings	Preferred stock
Balance at Dec. 31, 2019	$ 3,591.5	$ 3,561.1	$ 332.9	$ 929.5	$ 2,298.7	$ 30.4
Statements of equity
Net income attributed to common shareholder	118.7	118.7	0.0	0.0	118.7	0.0
Payment of dividends to parent	(60.0)	(60.0)	0.0	0.0	(60.0)	0.0
Stock-based compensation and other	0.5	0.5	0.0	0.5	0.0	0.0
Balance at Mar. 31, 2020	3,650.7	3,620.3	332.9	930.0	2,357.4	30.4
Balance at Dec. 31, 2019	3,591.5	3,561.1	332.9	929.5	2,298.7	30.4
Statements of equity
Net income attributed to common shareholder	189.3
Equity contribution from parent	65.0
Balance at Jun. 30, 2020	3,726.3	3,695.9	332.9	995.0	2,368.0	30.4
Balance at Mar. 31, 2020	3,650.7	3,620.3	332.9	930.0	2,357.4	30.4
Statements of equity
Net income attributed to common shareholder	70.6	70.6	0.0	0.0	70.6	0.0
Payment of dividends to parent	(60.0)	(60.0)	0.0	0.0	(60.0)	0.0
Equity contribution from parent	65.0	65.0	0.0	65.0	0.0	0.0
Balance at Jun. 30, 2020	3,726.3	3,695.9	332.9	995.0	2,368.0	30.4
Balance at Dec. 31, 2020	3,692.6	3,662.2	332.9	1,060.1	2,269.2	30.4
Statements of equity
Net income attributed to common shareholder	127.0	127.0	0.0	0.0	127.0	0.0
Payment of dividends to parent	(60.0)	(60.0)	0.0	0.0	(60.0)	0.0
Equity contribution from parent	30.0	30.0	0.0	30.0	0.0	0.0
Stock-based compensation and other	0.6	0.6	0.0	0.5	0.1	0.0
Balance at Mar. 31, 2021	3,790.2	3,759.8	332.9	1,090.6	2,336.3	30.4
Balance at Dec. 31, 2020	3,692.6	3,662.2	332.9	1,060.1	2,269.2	30.4
Statements of equity
Net income attributed to common shareholder	205.8
Equity contribution from parent	30.0
Balance at Jun. 30, 2021	3,809.0	3,778.6	332.9	1,090.7	2,355.0	30.4
Balance at Mar. 31, 2021	3,790.2	3,759.8	332.9	1,090.6	2,336.3	30.4
Statements of equity
Net income attributed to common shareholder	78.8	78.8	0.0	0.0	78.8	0.0
Payment of dividends to parent	(60.0)	(60.0)	0.0	0.0	(60.0)	0.0
Stock-based compensation and other	0.0	0.0	0.0	0.1	(0.1)	0.0
Balance at Jun. 30, 2021	$ 3,809.0	$ 3,778.6	$ 332.9	$ 1,090.7	$ 2,355.0	$ 30.4