Loss Reserves - Reconciliation of Changes in Loss Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loss Reserve [Roll Forward]
Reserve at beginning of year	$ 674,019	$ 985,635	$ 1,438,813
Less reinsurance recoverable	21,641	33,328	48,474	$ 50,493
Net reserve at beginning of year	533,693	640,691	937,161	$ 1,388,320
Losses incurred: [Abstract]
Current year	189,581	203,928	284,913
Prior years	(71,006)	(167,366)	(231,204)
Total losses incurred	118,575	36,562	53,709
Losses paid [Abstract]
Current year	4,018	7,298	11,267
Prior years	235,551	327,743	493,300
Reinsurance terminations	(13,996)	(2,009)	301
Total losses paid	225,573	333,032	504,868
Reserve at end of year	$ 555,334	$ 674,019	$ 985,635