Schedule of Business Acquisitions, by Acquisition (Details) - Interboro Insurance Company $ in Thousands	Apr. 29, 2016 USD ($)
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	10,841
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, Net	60,471
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 Liabilities	109
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	2,079
Payments to Acquire Businesses, Gross	48,450
Business Combination, Consideration Transferred, Other	8,550
Business Combination, Consideration Transferred, Liabilities Incurred	3,471
Business Combination, Consideration Transferred	$ 60,471