Schedule of Business Acquisitions, by Acquisition (Details) - USD ($) $ in Thousands		12 Months Ended
	Apr. 29, 2016	Dec. 31, 2017	Dec. 31, 2016
Interboro Insurance Company
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			$ 15,554
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities			66,527
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables			3,186
Business Combination, Assets Acquired and Liabilities Assumed, Reinsurance Receivable			1,042
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			5,877
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net			10,157
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other			3,980
Business Combination, Assets Acquired and Liabilities Assumed, Loss Reserves			(24,967)
Business Combination, Assets Acquired and Liabilities Assumed, Unearned Premiums			(26,243)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Deferred Revenue			(1,472)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles			8,334
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets			575
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other			(2,079)
Payments to Acquire Businesses, Gross	$ 48,450		$ 60,471
AmCo Holding Company
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents		$ 95,284
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities		222,920
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables		31,439
Business Combination, Assets Acquired and Liabilities Assumed, Reinsurance Receivable		20,230
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Prepaid Reinsurance Premiums		22,544
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		30,286
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net		59,475
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other		4,591
Business Combination, Assets Acquired and Liabilities Assumed, Loss Reserves		(60,529)
Business Combination, Assets Acquired and Liabilities Assumed, Unearned Premiums		(128,824)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Reinsurance Payable		(22,406)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		33,812
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities		(17,093)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other		(6,261)
Payments to Acquire Businesses, Gross		$ 285,468