Consolidated Statements of Equity - USD ($) $ in Millions	Total	Total Common Shareholders' Equity	Common Stock	Additional Paid-In Capital	Retained Earnings	Preferred Stock	UMERC transfer	UMERC transfer Total Common Shareholders' Equity	UMERC transfer Common Stock	UMERC transfer Additional Paid-In Capital	UMERC transfer Retained Earnings	UMERC transfer Preferred Stock	Transfer of investment in ATC	Transfer of investment in ATC Total Common Shareholders' Equity	Transfer of investment in ATC Common Stock	Transfer of investment in ATC Additional Paid-In Capital	Transfer of investment in ATC Retained Earnings	Transfer of investment in ATC Preferred Stock	Equity settlement of Bostco short-term note receivable	Equity settlement of Bostco short-term note receivable Total Common Shareholders' Equity	Equity settlement of Bostco short-term note receivable Common Stock	Equity settlement of Bostco short-term note receivable Additional Paid-In Capital	Equity settlement of Bostco short-term note receivable Retained Earnings	Equity settlement of Bostco short-term note receivable Preferred Stock
Equity
Cumulative effect adjustment from adoption of ASU 2016-09	$ 11.9	$ 11.9	$ 0.0	$ 0.0	$ 11.9	$ 0.0
Balance at Dec. 31, 2016	3,524.2	3,493.8	332.9	1,020.1	2,140.8	30.4
Equity
Net income attributed to common shareholder	335.6	335.6	0.0	0.0	335.6	0.0
Payment of dividends to parent	(240.0)	(240.0)	0.0	0.0	(240.0)	0.0
Equity contribution from parent	75.0	75.0	0.0	75.0	0.0	0.0
Transfer of net assets to UMERC							$ (61.1)	$ (61.1)	$ 0.0	$ (61.1)	$ 0.0	$ 0.0
Transfer of ATC ownership interest and related taxes													$ (228.6)	$ (228.6)	$ 0.0	$ (228.6)	$ 0.0	$ 0.0
Settlement of a short-tern note receivable between Bostco and our parent company																			$ (4.8)	$ (4.8)	$ 0.0	$ (4.8)	$ 0.0	$ 0.0
Stock-based compensation and other	2.1	2.1	0.0	2.1	0.0	0.0
Balance at Dec. 31, 2017	3,414.3	3,383.9	332.9	802.7	2,248.3	30.4
Equity
Net income attributed to common shareholder	358.3	358.3	0.0	0.0	358.3	0.0
Payment of dividends to parent	(310.0)	(310.0)	0.0	0.0	(310.0)	0.0
Equity contribution from parent	28.0	28.0	0.0	28.0	0.0	0.0
Stock-based compensation and other	0.6	0.6	0.0	0.6	0.0	0.0
Balance at Dec. 31, 2018	3,491.2	3,460.8	332.9	831.3	2,296.6	30.4
Equity
Net income attributed to common shareholder	362.1	362.1	0.0	0.0	362.1	0.0
Payment of dividends to parent	(360.0)	(360.0)	0.0	0.0	(360.0)	0.0
Equity contribution from parent	105.0	105.0	0.0	105.0	0.0	0.0
Transfer of net assets to UMERC							$ (7.3)	$ (7.3)	$ 0.0	$ (7.3)	$ 0.0	$ 0.0
Stock-based compensation and other	0.5	0.5	0.0	0.5	0.0	0.0
Balance at Dec. 31, 2019	$ 3,591.5	$ 3,561.1	$ 332.9	$ 929.5	$ 2,298.7	$ 30.4