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Financial Guarantee Liability - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Credit Risk [Member] | Fannie Mae and Freddie Mac [Member]
Guarantee Obligations [Line Items]
Outstanding principal balances of credit risk loans being serviced	$ 19,900.0	$ 18,800.0
Maximum potential loss of credit risk loans being serviced	5,600.0	5,300.0
Credit Risk [Member] | Fannie Mae and Freddie Mac [Member] | Credit Enhancement Agreement [Member]
Guarantee Obligations [Line Items]
Maximum potential loss of credit risk loans being serviced	$ 82.3	$ 1,200.0
Fannie Mae DUS or Freddie TAH Loans [Member] | Maximum [Member]
Guarantee Obligations [Line Items]
Percentage of contingent liability of actual losses incurred on outstanding loans	33.00%

X

- Definition

Percentage of contingent liability of actual losses incurred on outstanding loans.

+ References

No definition available.

+ Details

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- Definition

For entities that provide full or partial financing and servicing for customers supplementary to its major operations (such as a manufacturer or a homebuilder providing and servicing loans to buyers), represents the liabilities associated with such activities, including accounts payable and accrued liabilities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

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