Acquisitions (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands								12 Months Ended
	Nov. 29, 2019	Jun. 20, 2019	Nov. 14, 2018	Oct. 01, 2018	Nov. 17, 2017	Aug. 31, 2017	Apr. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period								11 years
Gross written premiums								$ 8,779,971	$ 7,864,469	$ 5,506,960
Ceded premiums written								$ 3,367,900	$ 3,076,891	$ 1,089,173
Customer Relationships [Member]
Business Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period								12 years
Trade Names [Member]
Business Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period								12 years
Other Intangible Assets [Member]
Business Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period								10 years
VSC Fire & Security, Inc [Member]
Business Acquisition [Line Items]
Total purchase consideration	$ 225,000
Cash consideration	204,000
Goodwill recognized	118,900
Intangible assets	$ 70,500
Percentage acquired	100.00%
VSC Fire & Security, Inc [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets	$ 54,000
Definite-lived intangible assets, weighted-average amortization period	12 years
VSC Fire & Security, Inc [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets	$ 14,000
Definite-lived intangible assets, weighted-average amortization period	12 years
VSC Fire & Security, Inc [Member] | Other Intangible Assets [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets	$ 2,500
Definite-lived intangible assets, weighted-average amortization period	8 years
Hagerty Group, LLC [Member]
Business Acquisition [Line Items]
Total purchase consideration		$ 212,500
Brahmin [Member]
Business Acquisition [Line Items]
Total purchase consideration				$ 192,900
Cash consideration				172,300
Goodwill recognized				63,800
Intangible assets				$ 93,300
Percentage acquired				90.00%
Noncontrolling interests				$ 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]
Total purchase consideration			$ 974,400
Cash consideration			974,400
Goodwill recognized			434,200
Goodwill, tax deductible			0
Intangible assets			$ 551,000
Percentage acquired			100.00%
Noncontrolling interests			$ 15,100
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
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
SureTec Financial Corp [Member]
Business Acquisition [Line Items]
Total purchase consideration							$ 246,900
Cash consideration							225,600
Goodwill recognized							70,400
Goodwill, tax deductible							0
Intangible assets							103,000
SureTec Financial Corp [Member] | Agent Relationships [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets							$ 92,000
Definite-lived intangible assets, weighted-average amortization period							15 years
Costa Farms [Member]
Business Acquisition [Line Items]
Total purchase consideration						$ 417,200
Cash consideration						387,900
Goodwill recognized						186,200
Payment of contingent consideration						29,300
Increase in the estimate of contingent consideration obligation						19,000
Intangible assets						$ 192,000
Percentage acquired						81.00%
Noncontrolling interests						$ 66,600
Costa Farms [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets						$ 161,000
Definite-lived intangible assets, weighted-average amortization period						17 years
Costa Farms [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets						$ 31,000
Definite-lived intangible assets, weighted-average amortization period						9 years
State National Companies, Inc. [Member]
Business Acquisition [Line Items]
Total purchase consideration					$ 918,800
Cash consideration					918,800
Goodwill recognized					379,200
Goodwill, tax deductible					0
Intangible assets					370,500
Finite-lived intangible assets					$ 338,500
Percentage acquired					100.00%
Cash State National stockholders will receive for each outstanding share of State National common stock					$ 21.00
State National Companies, Inc. [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets					$ 289,000
Definite-lived intangible assets, weighted-average amortization period					13 years
State National Companies, Inc. [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets					$ 22,500
Definite-lived intangible assets, weighted-average amortization period					13 years
State National Companies, Inc. [Member] | Technology [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets					$ 27,000
Definite-lived intangible assets, weighted-average amortization period					9 years
Hagerty Group, LLC [Member] | Hagerty Group, LLC [Member]
Business Acquisition [Line Items]
Gross written premiums								$ 422,100
Ceded premiums written								$ 202,100
Insurance Licenses [Member] | State National Companies, Inc. [Member]
Business Acquisition [Line Items]
Indefinite-lived intangible assets					$ 32,000