Loss Reserves - Prior Year Loss Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (147,658)	$ (110,302)	$ (100,359)
Decrease in estimated claim rate on primary defaults
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	(148,000)	(141,000)	(43,000)
Increase (decrease) in estimated severity on primary defaults
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	9,000	43,000	(35,000)
Change in estimates related to pool reserves, LAE reserves, reinsurance and other
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (9,000)	$ (12,000)	$ (22,000)