Loss Reserves - Reconciliation of Changes in Loss Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Loss Reserve [Roll Forward]
Reserve at beginning of year	$ 555,334	$ 674,019	$ 985,635
Less reinsurance recoverable	95,042	21,641	33,328	$ 48,474
Net reserve at beginning of year	785,495	533,693	640,691	$ 937,161
Losses incurred: [Abstract]
Current year	345,170	189,581	203,928
Prior years	19,604	(71,006)	(167,366)
Total losses incurred	364,774	118,575	36,562
Losses paid [Abstract]
Current year	3,069	4,018	7,298
Prior years	109,923	235,551	327,743
Reinsurance terminations	(20)	(13,996)	(2,009)
Total losses paid	112,972	225,573	333,032
Reserve at end of year	$ 880,537	$ 555,334	$ 674,019