Consolidated Balance Sheet - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	[1]
Statement of Financial Position [Abstract]
Cash and cash equivalents	$ 607.2	$ 681.2
Restricted cash	1,629.6	1,623.8
Premiums and fees receivable	4,857.5	4,082.8
Other current assets	1,024.4	881.6
Total current assets	8,118.7	7,269.4
Fixed assets - net	436.9	412.2
Deferred income taxes	806.2	851.6
Other noncurrent assets	573.6	567.1
Goodwill - net	4,625.6	4,164.8
Amortizable intangible assets - net	1,773.0	1,644.6
Total assets	16,334.0	14,909.7
Premiums payable to underwriting enterprises	5,740.2	4,986.0
Accrued compensation and other accrued liabilities	1,055.1	947.8
Deferred revenue - current	379.3	355.3
Premium financing borrowings	154.0	151.1
Corporate related borrowings - current	365.0	290.0
Total current liabilities	7,693.6	6,730.2
Corporate related borrowings - noncurrent	3,091.4	2,691.9
Deferred revenue - noncurrent	78.4	75.3
Other noncurrent liabilities	900.9	1,112.6
Total liabilities	11,764.3	10,610.0
Stockholders' equity:
Common stock - authorized 400.0 shares; issued and outstanding 184.0 shares in 2018 and 181.0 shares in 2017	184.0	181.0
Capital in excess of par value	3,541.9	3,388.2
Retained earnings	1,558.6	1,221.8
Accumulated other comprehensive loss	(785.6)	(555.4)
Stockholders' equity attributable to controlling interests	4,498.9	4,235.6
Stockholders' equity attributable to noncontrolling interests	70.8	64.1
Total stockholders' equity	4,569.7	4,299.7
Total liabilities and stockholders' equity	$ 16,334.0	$ 14,909.7

[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.