SCHEDULE VI - Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred policy acquisition costs	$ 62,006	$ 54,450	$ 72,365
Reserve for losses and loss adjustment expenses	2,045,506	1,661,459	1,292,349
Unearned premiums	524,377	386,473	418,114
Net earned premiums	823,746	815,398	741,109
Net investment income	75,652	61,256	61,119
Current year	603,094	582,604	533,905
Prior year	69,008	17,672	21,472
Total incurred losses and loss and adjustment expenses	672,102	600,276	555,377
Amortization of policy acquisition costs	83,893	111,103	116,001
Paid losses and loss adjustment expenses, net of reinsurance	488,729	396,013	326,680
Net written premiums	$ 896,150	$ 762,672	$ 766,626