EQUITY - Narrative (Details) $ / shares in Units, $ in Millions		3 Months Ended	12 Months Ended
	Jul. 18, 2024	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($) classOfCommonStockOutstanding $ / shares shares	Dec. 31, 2023 USD ($) $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Feb. 28, 2023 USD ($)	Dec. 31, 2021 shares
Class of Stock [Line Items]
Number of classes of common stock outstanding | classOfCommonStockOutstanding			3
Quarterly dividend on all classes of common stock (in dollars per share) | $ / shares			$ 1.40	$ 1.10	$ 0.95
Share expiration period			10 years
SARs exercise price (in dollars per share) | $ / shares			$ 198.54	$ 135.12	$ 120.38
Decrease in additional paid-in capital			$ 5.4	$ 3.1	$ 1.9
Intrinsic value of outstanding, exercisable and vested or excepted to vest restricted stock			28.4	35.0
Intrinsic value of shares exercised			4.6	$ 2.4	$ 1.1
Unrecognized compensation cost			$ 7.5
Weighted-average period of recognition			1 year 8 months 12 days
Weighted-average grant date fair value (in dollars per share) | $ / shares			$ 159.08	$ 114.68	$ 91.94
Compensation expense			$ 8.1	$ 6.8	$ 4.6
Granted (in dollars per share) | $ / shares			$ 197.52	$ 135.56	$ 120.14
Capital required for capital adequacy			$ 917.6	$ 862.6
Capital			1,500.0	1,500.0
Excess capital			583.5	589.4
SARs
Class of Stock [Line Items]
Tax benefit from exercise of SARs			$ 3.0	$ 1.7	$ 1.2
Outstanding, SARs (in shares) | shares			106,689	125,952	132,163		130,409
Stock option
Class of Stock [Line Items]
Outstanding, SARs (in shares) | shares			0
Minimum | Restricted Stock
Class of Stock [Line Items]
Vesting period			1 year	1 year	1 year
Maximum | Restricted Stock
Class of Stock [Line Items]
Vesting period			3 years	3 years	3 years
Class C non-voting common stock
Class of Stock [Line Items]
Remaining authorized amount for repurchase						$ 9.8
Shares repurchased (in shares) | shares			673,000
Shares repurchased			$ 19.8
Series C
Class of Stock [Line Items]
Preferred stock, dividend rate	6.00%		6.00%	6.00%	6.00%
Loss on retirement of preferred stock		$ 1.6