Loss Reserves (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) loan	Sep. 30, 2017 loan	Sep. 30, 2018 USD ($) loan	Sep. 30, 2017 USD ($) loan	Dec. 31, 2017 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Percentage of prior year delinquent inventory resolved (as a percent)			65.00%	59.00%
Prior years | $			$ 257,808	$ 407,977
Premium refund liability, expected claim payments | $	$ 44,000		44,000		$ 61,000
Prior years | $			$ (146,931)	$ (134,780)
Primary Delinquent Inventory [Roll Forward]
Delinquent inventory at the beginning of period (in loans) | loan	36,037	41,317	46,556	50,282
New notices (in loans) | loan	13,569	15,950	40,351	45,352
Cures (in loans) | loan	(14,197)	(13,546)	(47,620)	(45,382)
Paids (including those charged to a deductible or captive) (in loans) | loan	(1,374)	(2,195)	(4,446)	(7,403)
Rescissions and denials (in loans) | loan	(56)	(82)	(200)	(277)
Other items removed from inventory (in loans) | loan	(581)	(209)	(1,243)	(1,337)
Delinquent inventory at end of period (in loans) | loan	33,398	41,235	33,398	41,235
Settlements for commutations of coverage, pools of nonperforming loans, and claims paying practices
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years | $			$ 40,000	$ 54,000
Decrease in estimated claim rate on primary defaults
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years | $			(184,000)	(138,000)
Increase in estimated severity on primary defaults
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years | $			22,000	(2,000)
Change in estimates related to pool reserves, LAE reserves and reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years | $			$ 15,000	$ 5,000