Earnings Per Share (Computation of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2025	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Earnings Per Share [Abstract]
Net income	$ 142.5	$ 259.9	$ 273.9	$ 129.6	$ 805.8	$ 811.1	$ 591.2
Weighted-average shares outstanding - basic (in shares)					35.2	35.6	35.5
Add: Potential effect of diluted securities attributable to stock-based payments					0.2	0.2	0.2
Weighted-average shares outstanding - diluted (in shares)					35.4	35.8	35.7
Earnings per share – Basic:
Basic Earnings (Loss) per Share (in usd per share)	$ 4.08	$ 7.41	$ 7.75	$ 3.65	$ 22.89	$ 22.78	$ 16.65
Earnings per share – Diluted:
Diluted Earnings (Loss) per Share (in usd per share)	$ 4.07	$ 7.38	$ 7.71	$ 3.63	$ 22.79	$ 22.66	$ 16.58