Consolidated Balance Sheet - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Cash and cash equivalents	$ 664.6	$ 604.8
Restricted cash	2,909.7	2,019.1
Premiums and fees receivable	6,436.0	5,419.2
Other current assets	1,113.9	1,074.4
Total current assets	11,124.2	9,117.5
Fixed assets - net	450.7	467.4
Deferred income taxes	1,085.8	945.6
Other noncurrent assets	769.9	773.6
Right-of-use assets	373.9	393.5
Goodwill - net	6,127.0	5,618.5
Amortizable intangible assets - net	2,399.9	2,318.7
Total assets	22,331.4	19,634.8
Premiums payable to underwriting enterprises	7,784.6	6,348.5
Accrued compensation and other accrued liabilities	1,596.2	1,347.8
Deferred revenue - current	475.6	434.1
Premium financing borrowings	203.6	170.6
Corporate related borrowings - current	75.0	620.0
Total current liabilities	10,135.0	8,921.0
Corporate related borrowings - noncurrent	4,266.0	3,816.1
Deferred revenue - noncurrent	65.7	69.7
Lease liabilities - noncurrent	320.9	340.9
Other noncurrent liabilities	1,311.1	1,271.6
Total liabilities	16,098.7	14,419.3
Stockholders' equity:
Common stock - authorized 400.0 shares; issued and outstanding 193.7 shares in 2020 and 188.1 shares in 2019	193.7	188.1
Capital in excess of par value	4,264.4	3,825.7
Retained earnings	2,371.7	1,901.3
Accumulated other comprehensive loss	(643.6)	(759.6)
Stockholders' equity attributable to controlling interests	6,186.2	5,155.5
Stockholders' equity attributable to noncontrolling interests	46.5	60.0
Total stockholders' equity	6,232.7	5,215.5
Total liabilities and stockholders' equity	$ 22,331.4	$ 19,634.8