Consolidated Statement of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]
Net Income	$ 113.0
Balance, Beginning Balance at Dec. 31, 2017		$ 1,401.4	$ (28.2)	$ 695.5
Consolidated Statement of Shareholders' Equity [Roll Forward]
Common Stock Issued		19.7
Unrealized Gain (Loss) on Debt Securities	(0.1)		(0.1)
Defined Benefit Pension and Other Postretirement Plans	1.0		1.0
Common Stock Dividends	86.2			(86.2)
Balance, End of Period at Sep. 30, 2018	$ 2,116.1	1,421.1	(27.3)	722.3
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 1.68
Net Income	$ 30.7
Balance, Beginning Balance at Jun. 30, 2018		1,415.4	(27.6)	720.4
Consolidated Statement of Shareholders' Equity [Roll Forward]
Common Stock Issued		5.7
Unrealized Gain (Loss) on Debt Securities	0.0		0.0
Defined Benefit Pension and Other Postretirement Plans	0.3		0.3
Common Stock Dividends				(28.8)
Balance, End of Period at Sep. 30, 2018	$ 2,116.1	1,421.1	(27.3)	722.3
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.56
Net Income	$ 135.9
Balance, Beginning Balance at Dec. 31, 2018		1,428.5	(27.3)	754.6
Consolidated Statement of Shareholders' Equity [Roll Forward]
Common Stock Issued		6.6
Unrealized Gain (Loss) on Debt Securities	0.2		0.2
Defined Benefit Pension and Other Postretirement Plans	0.3		0.3
Common Stock Dividends	91.0			(91.0)
Balance, End of Period at Sep. 30, 2019	$ 2,207.8	1,435.1	(26.8)	799.5
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 1.7625
Net Income	$ 31.2
Balance, Beginning Balance at Jun. 30, 2019		1,433.3	(26.9)	798.6
Consolidated Statement of Shareholders' Equity [Roll Forward]
Common Stock Issued		1.8
Unrealized Gain (Loss) on Debt Securities	0.0		0.0
Defined Benefit Pension and Other Postretirement Plans	0.1		0.1
Common Stock Dividends				(30.3)
Balance, End of Period at Sep. 30, 2019	$ 2,207.8	$ 1,435.1	$ (26.8)	$ 799.5
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.5875