Consolidated Balance Sheet - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Cash and cash equivalents	$ 681.2	$ 545.5
Restricted cash	1,623.8	1,392.1
Premiums and fees receivable	2,157.2	1,844.8
Other current assets	708.4	633.7
Total current assets	5,170.6	4,416.1
Fixed assets - net	412.2	377.6
Deferred income taxes	905.1	796.5
Other noncurrent assets	567.0	504.3
Goodwill - net	4,197.9	3,767.8
Amortizable intangible assets - net	1,644.6	1,627.3
Total assets	12,897.4	11,489.6
Premiums payable to underwriting enterprises	3,475.9	2,996.1
Accrued compensation and other accrued liabilities	864.1	772.1
Unearned fees	74.8	69.0
Other current liabilities	56.4	70.9
Premium financing borrowings	151.1	125.6
Corporate related borrowings - current	290.0	578.0
Total current liabilities	4,912.3	4,611.7
Corporate related borrowings - noncurrent	2,691.9	2,144.6
Other noncurrent liabilities	1,128.3	1,077.5
Total liabilities	8,732.5	7,833.8
Stockholders' equity:
Common stock - authorized 400.0 shares; issued and outstanding 181.0 shares in 2017 and 178.3 shares in 2016	181.0	178.3
Capital in excess of par value	3,388.2	3,265.5
Retained earnings	1,095.9	916.4
Accumulated other comprehensive loss	(559.9)	(763.6)
Stockholders' equity attributable to controlling interests	4,105.2	3,596.6
Stockholders' equity attributable to noncontrolling interests	59.7	59.2
Total stockholders' equity	4,164.9	3,655.8
Total liabilities and stockholders' equity	$ 12,897.4	$ 11,489.6