Earnings Per Common Share - Net Income, Weighted Average Common Shares Used in Calculating Basic Earnings Per Common Share and Diluted EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Numerator
Net income	$ 156.3	$ 52.2	$ 269.9	$ 201.2
Less: Common stock dividends paid	(39.7)	(38.6)	(76.7)	(76.0)
Undistributed earnings	116.6	13.6	193.2	125.2
Undistributed earnings	$ 116.6	$ 13.6	$ 193.2	$ 125.2
Denominator
Weighted average common shares outstanding used in basic per common share calculations (in shares)	53,745,611	54,607,321	53,619,711	55,190,104
Incremental common shares from:
Weighted average common shares outstanding used in diluted per common share calculations (in shares)	53,889,682	55,014,947	53,843,035	55,663,946
Basic
Distributed earnings (in dollars per share)	$ 0.74	$ 0.71	$ 1.43	$ 1.38
Undistributed earnings (in dollars per share)	2.17	0.25	3.60	2.27
Net income (in dollars per share)	2.91	0.96	5.03	3.65
Diluted
Distributed earnings (in dollars per share)	0.74	0.70	1.42	1.36
Undistributed earning (in dollars per share)	2.16	0.25	3.59	2.25
Net income (in dollars per share)	$ 2.90	$ 0.95	$ 5.01	$ 3.61
PSUs
Incremental common shares from:
Incremental common shares (in shares)	78,906	361,287	158,159	427,503
ESPP
Incremental common shares from:
Incremental common shares (in shares)	65,165	46,339	65,165	46,339