Consolidated Statement of Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]	Non-Controlling Interest in Subsidiaries [Member]
Consolidated Statement of Equity [Roll Forward]
Adjustments to Opening Balance – Net of Income Taxes (a)			$ 0.0	$ 0.0
Balance, Beginning of Period at Dec. 31, 2016		$ 1,295.3	(28.2)	625.9	$ 0.0
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		106.1
Unrealized Gain (Loss) on Securities	$ 0.9		0.9
Defined Benefit Pension and Other Postretirement Plans	4.7		4.7
Net Income	172.2			172.2	0.0
Common Stock Dividends	108.7			(108.7)
Proceeds from Noncontrolling Interests	0.0
Balance, Ending of Period at Dec. 31, 2017	$ 2,068.2	1,401.4	(22.6)	689.4	0.0
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 2.14
Adjustments to Opening Balance – Net of Income Taxes (a)			(5.6)	6.1
Common Stock Issued		27.1
Unrealized Gain (Loss) on Securities	$ (0.1)		(0.1)
Defined Benefit Pension and Other Postretirement Plans	1.0		1.0
Net Income	174.1			174.1	0.0
Common Stock Dividends	115.0			(115.0)
Proceeds from Noncontrolling Interests	0.0
Balance, Ending of Period at Dec. 31, 2018	$ 2,155.8	1,428.5	(27.3)	754.6	0.0
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 2.24
Adjustments to Opening Balance – Net of Income Taxes (a)			0.0	0.0
Common Stock Issued		8.2
Unrealized Gain (Loss) on Securities	$ 0.2		0.2
Defined Benefit Pension and Other Postretirement Plans	3.5		3.5
Net Income	185.5			185.6	(0.1)
Common Stock Dividends	121.4			(121.4)
Proceeds from Noncontrolling Interests	(103.8)
Balance, Ending of Period at Dec. 31, 2019	$ 2,335.6	$ 1,436.7	$ (23.6)	$ 818.8	$ 103.7
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 2.35