Reserve for Losses and Loss Adjustment Expenses - Reconciliation of Losses and Loss Adjustment Balances (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
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	$ 1,351,689	$ 1,221,652	$ 1,377,461	$ 1,194,088
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	97,620	144,738	193,602	283,697
Prior years	1,126	2,315	2,000	3,283
Total incurred losses and loss and adjustment expenses	98,746	147,053	195,602	286,980
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	2,337	20,575	6,609	25,254
Prior years	108,564	110,200	226,920	217,884
Total loss and loss adjustment expense payments	110,901	130,775	233,529	243,138
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	$ 1,351,689	$ 1,221,652	$ 1,377,461	$ 1,194,088	$ 1,339,534	$ 1,377,461	$ 1,237,930
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period					727,437		545,404
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period					$ 2,066,971	$ 2,045,506	$ 1,783,334