Loss Reserves - Prior Year Loss Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (231,204)	$ (147,658)	$ (110,302)
Decrease in estimated claim rate on primary delinquencies
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	(248,000)	(148,000)	(141,000)
Increase in estimated severity on primary delinquencies
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	9,000	9,000	43,000
Change in estimates related to pool reserves, LAE reserves, reinsurance and other
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 8,000	$ (9,000)	$ (12,000)