Mortgage-Backed Securities - Aggregate Other Than Temporary Impairments (Detail) - Residential Mortgage-Backed Securities - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Total other-than-temporary impairment losses	$ (9,589)	$ (8,700)	$ (8,713)
Portion of loss recognized in other comprehensive income (loss)	(48,398)	(58,744)	(55,279)
Net other-than-temporary credit impairment losses	$ (57,987)	$ (67,444)	$ (63,992)

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.

No definition available.

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- Definition

The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

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- Definition

Amount before tax of other than temporary impairment (OTTI) on investment in available-for-sale debt security, recognized in other comprehensive loss, attributable to parent entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669646-108580

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25362-111560

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- Definition

The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

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