Loss Reserves - Reconciliation of Changes in Loss Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss Reserve [Roll Forward]
Reserve at beginning of year	$ 1,893,402	$ 2,396,807	$ 3,061,401
Less reinsurance recoverable	50,493	44,487	57,841	$ 64,085
Net reserve at beginning of year	1,388,320	1,848,915	2,338,966	$ 2,997,316
Losses incurred: [Abstract]
Current year	387,815	453,849	596,436
Prior years	(147,658)	(110,302)	(100,359)
Total losses incurred	240,157	343,547	496,077
Losses paid [Abstract]
Current year	14,823	25,980	32,919
Prior years	689,258	823,058	1,121,508
Reinsurance terminations	(3,329)	(15,440)	0
Total losses paid	700,752	833,598	1,154,427
Reserve at end of year	$ 1,438,813	$ 1,893,402	$ 2,396,807