Consolidated Statement of Shareholders' Equity Consolidated Statement of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]
Balance, Beginning Balance at Dec. 31, 2017		$ 1,401.4	$ (28.2)	$ 695.5
Consolidated Statement of Shareholders' Equity [Roll Forward]
Common Stock Issued		14.0
Unrealized Gain (Loss) on Debt Securities	$ (0.1)		(0.1)
Defined Benefit Pension and Other Postretirement Plans	0.7		0.7
Net Income				82.3
Common Stock Dividends	57.4			(57.4)
Balance, End of Period at Jun. 30, 2018	$ 2,108.2	1,415.4	(27.6)	720.4
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 1.12
Balance, Beginning Balance at Mar. 31, 2018		1,407.4	(27.9)	717.8
Consolidated Statement of Shareholders' Equity [Roll Forward]
Common Stock Issued		8.0
Unrealized Gain (Loss) on Debt Securities	$ 0.0		0.0
Defined Benefit Pension and Other Postretirement Plans	0.3		0.3
Net Income				31.3
Common Stock Dividends				(28.7)
Balance, End of Period at Jun. 30, 2018	$ 2,108.2	1,415.4	(27.6)	720.4
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.56
Balance, Beginning Balance at Dec. 31, 2018		1,428.5	(27.3)	754.6
Consolidated Statement of Shareholders' Equity [Roll Forward]
Common Stock Issued		4.8
Unrealized Gain (Loss) on Debt Securities	$ 0.2		0.2
Defined Benefit Pension and Other Postretirement Plans	0.2		0.2
Net Income				104.7
Common Stock Dividends	60.7			(60.7)
Balance, End of Period at Jun. 30, 2019	$ 2,205.0	1,433.3	(26.9)	798.6
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 1.175
Balance, Beginning Balance at Mar. 31, 2019		1,431.1	(27.2)	794.8
Consolidated Statement of Shareholders' Equity [Roll Forward]
Common Stock Issued		2.2
Unrealized Gain (Loss) on Debt Securities	$ 0.1		0.1
Defined Benefit Pension and Other Postretirement Plans	0.2		0.2
Net Income				34.2
Common Stock Dividends				(30.4)
Balance, End of Period at Jun. 30, 2019	$ 2,205.0	$ 1,433.3	$ (26.9)	$ 798.6
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.5875