Reserve for Losses and Loss Adjustment Expenses - Reconciliation of Losses and Loss Adjustment Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) [Abstract]
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at beginning of period	$ 1,237,930	$ 1,092,818	$ 1,194,088	$ 989,825
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	157,113	138,168	440,810	436,921
Prior years	56,971	12,219	60,254	11,833
Total incurred losses and loss and adjustment expenses	214,084	150,387	501,064	448,754
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	21,021	25,149	46,275	57,903
Prior years	104,513	72,695	322,397	235,315
Total loss and loss adjustment expense payments	125,534	97,844	368,672	293,218
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	$ 1,237,930	$ 1,092,818	$ 1,194,088	$ 989,825	$ 1,326,480	$ 1,194,088	$ 1,145,361
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period					614,827	467,371	424,400
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period					$ 1,941,307	$ 1,661,459	$ 1,569,761