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Acquisitions Pro Forma (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									4 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Apr. 03, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Apr. 29, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues											$ 654,420	$ 487,117	$ 357,569
Net Income (Loss) Attributable to Parent	$ 27,001	$ (28,012)	$ 7,257		$ 3,899	$ (10,517)	$ 3,423	$ 9,841	$ 2,951		$ 10,145	$ 5,698	$ 27,358
Earnings Per Share, Diluted	$ 0.63	$ (0.66)	$ 0.17		$ 0.18	$ (0.49)	$ 0.16	$ 0.45	$ 0.14		$ 0.27	$ 0.26	$ 1.28
Interboro Insurance Company
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues										$ 18,963
Business Acquisition, Pro Forma Revenue												$ 506,080
Business Acquisition, Pro Forma Net Income (Loss)												$ 13,885
Business Acquisition, Pro Forma Earnings Per Share, Diluted												$ 0.64
Net Income (Loss) Attributable to Parent										$ 8,187
Earnings Per Share, Diluted										$ 0.38
AmCo Holding Company
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues				$ 38,096								$ 175,032
Business Acquisition, Pro Forma Revenue											$ 692,516	662,149
Business Acquisition, Pro Forma Net Income (Loss)											$ 16,857	$ 37,658
Business Acquisition, Pro Forma Earnings Per Share, Diluted											$ 0.39	$ 0.88
Net Income (Loss) Attributable to Parent				$ 6,712								$ 31,960
Earnings Per Share, Diluted				$ 0.00								$ 0.00