Acquisitions (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands						3 Months Ended	12 Months Ended
	Nov. 14, 2018	Oct. 01, 2018	Nov. 17, 2017	Aug. 31, 2017	Apr. 30, 2017	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period							15 years
Agent Relationships [Member]
Business Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period							12 years
Customer Relationships [Member]
Business Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period							16 years
Trade Names [Member]
Business Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period							15 years
Other Intangible Assets [Member]
Business Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period							5 years
Investment Management Agreements [Member]
Business Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period							16 years
Brahmin [Member]
Business Acquisition [Line Items]
Total purchase consideration		$ 193,800
Cash consideration		173,300
Goodwill recognized		72,900
Intangible assets		$ 81,300
Percentage acquired		90.00%
Non-controlling interests		$ 19,600
Brahmin [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets		$ 45,000
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	$ 972,600
Cash consideration	972,600
Goodwill recognized	474,900
Goodwill, tax deductible	0
Intangible assets	$ 516,800
Percentage acquired	100.00%
Non-controlling interests	$ 23,400
Nephila Holdings Limited [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets	$ 22,000
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	$ 441,000
Definite-lived intangible assets, weighted-average amortization period	16 years
Nephila Holdings Limited [Member] | Broker Relationships [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets	$ 31,000
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	$ 22,800
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
Payment Of Contingent Consideration				29,300
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability				19,000
Goodwill recognized				186,200
Intangible assets				$ 192,000
Percentage acquired				81.00%
Non-controlling 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			379,150			$ 379,200
Goodwill, tax deductible						0
Intangible assets			$ 370,500
Finite-lived intangible assets						$ 338,500
Definite-lived intangible assets, weighted-average amortization period						13 years
Percentage acquired			100.00%
Cash State National stockholders will receive for each outstanding share of State National common stock			$ 21.00
Unpaid losses and loss adjustment expenses acquired, adjustment to fair value			$ 64,500
Unpaid losses and loss adjustment expenses, weighted average amortization period, years			3 years
Unpaid losses and loss adjustment expenses, unamortized fair value adjustment							$ 35,300	$ 57,700
Deferred ceding fees acquired, adjustment to fair value			$ 28,300
Deferred ceding fees, weighted average amortization period, years			1 year
Deferred ceding fees, unamortized fair value adjustment							$ 0	$ 19,300
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets						$ 32,000
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