CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net income	$ 870.5	$ 671.8	$ 587.1
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	166.0	146.6	119.6
Depreciation	40.0	39.2	33.3
Non-cash stock-based compensation	89.4	66.1	61.0
Change in estimated acquisition earn-out payables	21.8	(38.9)	40.4
Deferred income taxes	12.3	42.8	33.6
Amortization of debt discount and disposal of deferred financing costs	1.0	3.8	2.8
Amortization of discounts and premiums, investment	3.5	0.2	0.1
Net (gain)/loss on sales/disposals of businesses, investments, fixed assets and customer accounts	(140.3)	(3.6)	(7.1)
Payments on acquisition earn-outs in excess of original estimated payables	(29.3)	(30.1)	(21.1)
Effect of changes in foreign exchange rate changes	0.2	(0.6)	0.5
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Commissions, fees and other receivables (increase) decrease	(106.4)	(60.9)	(61.9)
Reinsurance recoverable (increase) decrease	705.9	(767.9)	(19.6)
Prepaid reinsurance premiums (increase) decrease	(68.3)	(1.0)	(14.6)
Other assets (increase) decrease	(117.7)	(17.6)	(53.7)
Losses and loss adjustment reserve increase (decrease)	(709.6)	777.8	19.6
Unearned premiums increase (decrease)	50.1	20.1	14.6
Accounts payable increase (decrease)	260.0	124.3	54.4
Accrued expenses and other liabilities increase (decrease)	43.2	37.0	66.9
Other liabilities increase (decrease)	(82.8)	(127.7)	(47.1)
Net cash provided by operating activities	1,009.5	881.4	808.8
Cash flows from investing activities:
Additions to fixed assets	(68.9)	(52.6)	(45.0)
Payments for businesses acquired, net of cash acquired	(630.7)	(1,927.7)	(366.8)
Proceeds from sales of businesses, fixed assets and customer accounts	106.6	60.4	16.6
Purchases of investments	(7.2)	(0.1)	(12.4)
Proceeds from sales of investments	13.2	7.4	10.8
Net cash used in investing activities	(587.0)	(1,912.6)	(396.8)
Cash flows from financing activities:
Fiduciary receivables and liabilities, net	188.5	96.2	133.7
Deferred acquisition purchase payment		(5.1)
Payments on acquisition earn-outs	(89.5)	(76.2)	(62.5)
Proceeds from long-term debt		2,000.0
Payments on long-term debt	(250.6)	(61.3)	(73.1)
Deferred debt issuance costs		(23.4)	(2.6)
Borrowings on revolving credit facility	420.0	350.0
Payments on revolving credit facilities	(320.0)	(350.0)
Issuances of common stock for employee stock benefit plans	39.8	37.6	34.0
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(39.8)	(48.8)	(49.8)
Purchase of treasury stock	(0.1)	(74.1)	(82.6)
Cash dividends paid	(135.0)	(119.5)	(107.2)
Net cash (used in) provided by financing activities	(186.7)	1,725.4	(210.1)
Effect of foreign exchange rate changes on cash and cash equivalents inclusive of fiduciary cash	33.9	(131.2)	(3.6)
Net increase in cash and cash equivalents inclusive of fiduciary cash	269.7	563.0	198.3
Cash and cash equivalents inclusive of fiduciary cash at beginning of period	2,033.2	1,470.2	1,271.9
Cash and cash equivalents inclusive of fiduciary cash at end of period	$ 2,302.9	$ 2,033.2	$ 1,470.2