Loss Reserves - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan	Dec. 31, 2015 USD ($) loan
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Premium refund liability, expected claim payments	$ 61,000	$ 85,000
Other items removed from inventory | loan	1,337	1,273	1,121
Losses for unpaid claims and adjustment expense	$ 493,300	$ 689,258	$ 823,058
Percent of inventory in default for more than 36 consecutive months	45.00%	47.00%	50.00%
Pool insurance notice inventory (in number of loans) | loan	1,309	1,883	2,739
Settlements for commutations of coverage, pools of nonperforming loans
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses for unpaid claims and adjustment expense	$ 54,000
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]
Losses for unpaid claims and adjustment expense		$ 53,000
Payments for settlements
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses for unpaid claims and adjustment expense			$ 10,000
Payments for settlements | Freddie Mac
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses for unpaid claims and adjustment expense		$ 42,000	$ 42,000
Decrease in estimated claim rate on primary delinquencies
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Percentage of prior year default inventory resolved (in hundredths)	67.00%	63.00%	60.00%