Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	199,937,543			207,021,554			202,614,386	207,611,420
Net Income	$ 464.9	$ 896.1	$ 214.7	$ 327.8	$ 331.8	$ 334.6	$ 1,575.7	$ 994.2
Less: Dividends on Preferred Stock	16.2			16.2			37.1	37.1
Net Income Applicable to Common Stock	448.7			311.6			1,538.6	957.1
Less: Earnings Allocated to Participating Securities	3.6			3.1			13.2	9.7
Earnings Allocated to Common Shares Outstanding	$ 445.1			$ 308.5			$ 1,525.4	$ 947.4
Basic Net Income Per Common Share (in dollars per share)	$ 2.23			$ 1.49			$ 7.53	$ 4.56
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	199,937,543			207,021,554			202,614,386	207,611,420
Plus: Dilutive Effect of Share-based Compensation (in shares)	611,520			231,438			517,014	316,300
Average Common and Potential Common Shares (in shares)	200,549,063			207,252,992			203,131,400	207,927,720
Earnings Allocated to Common and Potential Common Shares	$ 445.1			$ 308.5			$ 1,525.4	$ 947.3
Diluted Net Income Per Common Share (in dollars per share)	$ 2.22			$ 1.49			$ 7.51	$ 4.56
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0			0			200,000	100,000