EQUITY - Narrative (Details) $ / shares in Units, $ in Millions		12 Months Ended			24 Months Ended
	Dec. 31, 2023 USD ($) shares	Dec. 31, 2023 USD ($) classOfCommonStockOutstanding $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) $ / shares shares	Mar. 10, 2023 shares	Mar. 31, 2023 USD ($)	Mar. 10, 2021 USD ($)	Dec. 31, 2020 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		$ 1.10	$ 0.95	$ 0.88
Share expiration period		10 years
SARs exercise price (in dollars per share) | $ / shares		$ 135.12	$ 120.38	$ 88.68
Decrease in additional paid-in capital		$ 3.1	$ 1.9	$ 1.3
Intrinsic value of outstanding, exercisable and vested or excepted to vest restricted stock	$ 35.0	35.0	16.3
Intrinsic value of shares exercised		2.4	$ 1.1	$ 0.9
Unrecognized compensation cost	7.7	$ 7.7
Weighted-average period of recognition		1 year 10 months 24 days
Weighted-average grant date fair value (in dollars per share) | $ / shares		$ 114.68	$ 91.94	$ 65.48
Compensation expense		$ 6.8	$ 4.6	$ 4.3
Granted (in dollars per share) | $ / shares		$ 135.56	$ 120.14	$ 88.92
Capital required for capital adequacy	862.6	$ 862.6	$ 805.9
Capital	1,500.0	1,500.0	1,300.0
Excess capital	$ 589.4	589.4	516.9
SARs
Class of Stock [Line Items]
Tax benefit from exercise of SARs		$ 1.7	$ 1.2	$ 0.9
Outstanding, SARs (in shares) | shares	125,952	125,952	132,163	130,409				116,417
Stock option
Class of Stock [Line Items]
Outstanding, SARs (in shares) | shares	0	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]
Shares repurchased (in shares) | shares	673,000	0			0
Shares repurchased	$ 19.8
Remaining authorized amount for repurchase						$ 9.8	$ 9.8
Stock repurchase, period					2 years