Loss Reserves - Reconciliation of Changes in Loss Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Loss Reserve [Roll Forward]
Reserve at beginning of year	$ 1,438,813	$ 1,893,402	$ 2,396,807
Less reinsurance recoverable	48,474	50,493	44,487	$ 57,841
Net reserve at beginning of year	937,161	1,388,320	1,848,915	$ 2,338,966
Losses incurred: [Abstract]
Current year	284,913	387,815	453,849
Prior years	(231,204)	(147,658)	(110,302)
Total losses incurred	53,709	240,157	343,547
Losses paid [Abstract]
Current year	11,267	14,823	25,980
Prior years	493,300	689,258	823,058
Reinsurance terminations	301	(3,329)	(15,440)
Total losses paid	504,868	700,752	833,598
Reserve at end of year	$ 985,635	$ 1,438,813	$ 1,893,402