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	303.9
Equity contribution from parent	130.0
Balance at Sep. 30, 2020	3,846.0	3,815.6	332.9	1,060.1	2,422.6	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
Statements of equity
Net income attributed to common shareholder	114.6	114.6	0.0	0.0	114.6	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
Stock-based compensation and other	0.1	0.1	0.0	0.1	0.0	0.0
Balance at Sep. 30, 2020	3,846.0	3,815.6	332.9	1,060.1	2,422.6	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	335.3
Equity contribution from parent	230.0
Balance at Sep. 30, 2021	4,018.6	3,988.2	332.9	1,290.8	2,364.5	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
Statements of equity
Net income attributed to common shareholder	129.5	129.5	0.0	0.0	129.5	0.0
Payment of dividends to parent	(120.0)	(120.0)	0.0	0.0	(120.0)	0.0
Equity contribution from parent	200.0	200.0	0.0	200.0	0.0	0.0
Stock-based compensation and other	0.1	0.1	0.0	0.1	0.0	0.0
Balance at Sep. 30, 2021	$ 4,018.6	$ 3,988.2	$ 332.9	$ 1,290.8	$ 2,364.5	$ 30.4