Significant Transactions - NSM (Details) £ in Millions							3 Months Ended		12 Months Ended
	Apr. 07, 2020 USD ($)	Apr. 07, 2020 GBP (£)	Jun. 28, 2019 USD ($)	Apr. 01, 2019 USD ($)	Dec. 03, 2018 USD ($)	May 11, 2018 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2019 USD ($) earnout_period	Dec. 31, 2018 USD ($)	Apr. 07, 2020 GBP (£)	May 18, 2018 USD ($)	Feb. 05, 2018 USD ($)
Other Significant Noncash Transactions [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired									$ 127,300,000		$ 258,000,000.0	$ 295,200,000
Equity method investment, aggregate cost															$ 125,000,000.0
Change in fair value of contingent consideration earnout liabilities									(3,300,000)		2,100,000	$ 2,700,000
Business Combination, Earnout Period Payments									6,400,000
Goodwill									525,500,000		394,700,000
Embrace Pet Insurance
Other Significant Noncash Transactions [Line Items]
Goodwill and intangible assets acquired				$ 67,600,000
Intangible assets acquired				15,400,000
Goodwill				52,200,000
Kingsbridge Group Limited
Other Significant Noncash Transactions [Line Items]
Goodwill and intangible assets acquired	$ 131,700,000
NSM
Other Significant Noncash Transactions [Line Items]
Long-term line of credit						$ 100,000,000.0
Contingent consideration earnout liabilities									14,600,000		20,600,000
Change in fair value of contingent consideration earnout liabilities									(3,300,000)		2,100,000
NSM | KBK Insurance Group, Inc. & KBK Premium Services, Inc.
Other Significant Noncash Transactions [Line Items]
Contingent consideration earnout liabilities									7,000,000.0		7,600,000
NSM | Embrace Pet Insurance
Other Significant Noncash Transactions [Line Items]
Additional investment in acquisition				58,200,000
Proceeds from borrowings				$ 20,400,000
Percentage of voting interests acquired				100.00%
Payments to acquire businesses, gross				$ 71,500,000
NSM | Kingsbridge Group Limited
Other Significant Noncash Transactions [Line Items]
Contingent consideration earnout liabilities	4,100,000								300,000
Equity method investment, aggregate cost	132,200,000												£ 107.2
Additional investment in acquisition	80,300,000
Proceeds from borrowings	$ 52,400,000	£ 42.5
Percentage of voting interests acquired	100.00%												100.00%
Acquisitions of businesses	$ 111,500,000
Finite-lived Intangible Assets Acquired	20,200,000
Pretax income	4,100,000
Foreign currency translation gain	300,000
NSM | NSM Bank Facility
Other Significant Noncash Transactions [Line Items]
Long-term line of credit									277,400,000
NSM | NSM Bank Facility | Term Loan
Other Significant Noncash Transactions [Line Items]
Long-term line of credit	276,400,000								277,400,000		221,300,000			$ 51,000,000.0
Proceeds from borrowings	$ 52,400,000	£ 42.5							$ 52,400,000	£ 42.5	42,900,000
NSM | NSM Bank Facility | Term Loan | Embrace Pet Insurance
Other Significant Noncash Transactions [Line Items]
Proceeds from borrowings											$ 20,400,000
NSM
Other Significant Noncash Transactions [Line Items]
Ownership interest (as a percent)						95.00%			96.60%		96.40%
Fully diluted ownership interest (as a percent)						83.60%			89.60%		88.40%
Purchases of consolidated subsidiaries, net of cash acquired						$ 274,200,000
Purchase price adjustment								$ 2,100,000
Assets acquired						495,200,000
Goodwill and intangible assets acquired						383,000,000.0
Liabilities acquired						204,600,000
Contingent consideration earnout liabilities						10,200,000
Noncontrolling interest acquired						14,400,000
Transaction costs						$ 6,300,000
Number of earnout periods | earnout_period											2
Fresh Insurance Services Group Limited
Other Significant Noncash Transactions [Line Items]
Ownership interest (as a percent)														100.00%
Purchase price adjustment											$ 700,000
Assets acquired														$ 72,600,000
Goodwill and intangible assets acquired														54,600,000
Liabilities acquired														22,300,000
Contingent consideration earnout liabilities														7,500,000
Equity method investment, aggregate cost														$ 49,600,000
KBK Insurance Group, Inc. & KBK Premium Services, Inc.
Other Significant Noncash Transactions [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired					$ 60,000,000.0
Goodwill and intangible assets acquired					59,400,000						32,600,000
Additional investment in acquisition					29,000,000.0
Proceeds from borrowings					$ 30,100,000
Intangible assets acquired											$ 32,700,000
Change in fair value of contingent consideration earnout liabilities							$ 5,900,000
AIG Collector Car Business
Other Significant Noncash Transactions [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired			$ 82,500,000
Additional investment in acquisition			59,100,000
Intangible assets acquired			82,500,000
AIG Collector Car Business | NSM
Other Significant Noncash Transactions [Line Items]
Proceeds from borrowings			$ 22,500,000