Consolidated Balance Sheet - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Statement Of Financial Position [Abstract]
Cash and cash equivalents	$ 528.6	$ 402.6
Restricted cash	4,110.1	4,063.7
Premiums and fees receivable	21,608.1	11,753.1
Other current assets	1,272.5	1,451.0
Total current assets	27,519.3	17,670.4
Fixed assets - net	505.1	500.8
Deferred income taxes	1,169.4	1,228.5
Other noncurrent assets	980.2	966.5
Right-of-use assets	382.2	358.6
Goodwill	8,727.7	8,666.2
Amortizable intangible assets - net	3,877.6	3,954.0
Total assets	43,161.5	33,345.0
Premiums payable to underwriting enterprises	23,175.4	13,845.6
Accrued compensation and other current liabilities	1,968.2	1,895.1
Deferred revenue - current	577.8	520.9
Premium financing debt	173.4	228.4
Corporate related borrowings - current	250.0	245.0
Total current liabilities	26,144.8	16,735.0
Corporate related borrowings - noncurrent	5,760.9	5,810.2
Deferred revenue - noncurrent	64.5	58.7
Lease liabilities - noncurrent	333.3	309.3
Other noncurrent liabilities	1,814.9	1,871.7
Total liabilities	34,118.4	24,784.9
Stockholders' equity:
Common stock - issued and outstanding 210.1 shares in 2022 and 208.5 shares in 2021	210.1	208.5
Capital in excess of par value	6,232.0	6,143.7
Retained earnings	3,213.0	2,882.3
Accumulated other comprehensive loss	(665.7)	(726.1)
Stockholders' equity attributable to controlling interests	8,989.4	8,508.4
Stockholders' equity attributable to noncontrolling interests	53.7	51.7
Total stockholders' equity	9,043.1	8,560.1
Total liabilities and stockholders' equity	$ 43,161.5	$ 33,345.0