Loss Reserves - Reconciliation of Changes in Loss Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Reserve [Roll Forward]
Reserve at beginning of year	$ 985,635	$ 1,438,813	$ 1,893,402
Less reinsurance recoverable	33,328	48,474	50,493	$ 44,487
Net reserve at beginning of year	640,691	937,161	1,388,320	$ 1,848,915
Losses incurred: [Abstract]
Current year	203,928	284,913	387,815
Prior years	(167,366)	(231,204)	(147,658)
Total losses incurred	36,562	53,709	240,157
Losses paid [Abstract]
Current year	7,298	11,267	14,823
Prior years	327,743	493,300	689,258
Reinsurance terminations	(2,009)	301	(3,329)
Total losses paid	333,032	504,868	700,752
Reserve at end of year	$ 674,019	$ 985,635	$ 1,438,813