Acquisitions (Narrative) (Details) - USD ($) $ in Thousands				3 Months Ended		9 Months Ended
	Jun. 20, 2019	Nov. 14, 2018	Oct. 01, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Business Acquisition [Line Items]
Gross premium volume				$ 2,264,492	$ 2,043,771	$ 6,766,681	$ 6,088,088
Ceded premiums written				894,108	$ 827,500	2,613,566	$ 2,356,409
Hagerty Group, LLC [Member]
Business Acquisition [Line Items]
Total purchase consideration	$ 212,500
Brahmin [Member]
Business Acquisition [Line Items]
Percentage acquired			90.00%
Total purchase consideration			$ 192,900
Cash consideration			172,300
Goodwill recognized			63,800
Intangible Assets			93,300
Noncontrolling Interest			19,600
Brahmin [Member] | Customer relationships [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets			$ 57,000
Definite-lived intangible assets, weighted-average amortization period			16 years
Brahmin [Member] | Trade names [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets			$ 35,000
Definite-lived intangible assets, weighted-average amortization period			16 years
Brahmin [Member] | Other intangible assets [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets			$ 1,300
Definite-lived intangible assets, weighted-average amortization period			8 years
Nephila Holdings Limited [Member]
Business Acquisition [Line Items]
Percentage acquired		100.00%
Total purchase consideration		$ 974,400
Cash consideration		974,400
Goodwill recognized		434,200
Intangible Assets		551,000
Goodwill, tax deductible		0
Noncontrolling Interest		15,100
Nephila Holdings Limited [Member] | Investment management agreements [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets		$ 468,000
Definite-lived intangible assets, weighted-average amortization period		17 years
Nephila Holdings Limited [Member] | Broker relationships [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets		$ 32,000
Definite-lived intangible assets, weighted-average amortization period		12 years
Nephila Holdings Limited [Member] | Technology-based intangible assets [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets		$ 27,000
Definite-lived intangible assets, weighted-average amortization period		6 years
Nephila Holdings Limited [Member] | Trade names [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets		$ 24,000
Definite-lived intangible assets, weighted-average amortization period		14 years
Hagerty Group, LLC [Member] | Hagerty Group, LLC [Member]
Business Acquisition [Line Items]
Gross premium volume				120,800		333,700
Ceded premiums written				$ 57,700		$ 159,800