Mortgage-Backed Securities - Aggregate Other Than Temporary Impairments (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Total other-than-temporary impairment losses	$ (805)	$ (749)	$ (1,099)	$ (3,462)
Portion of loss recognized in other comprehensive income	(8,326)	(12,760)	(9,190)	(28,748)
Net other-than-temporary credit impairment losses	(9,131)	(13,509)	(10,289)	(32,210)
Residential Mortgage-Backed Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Total other-than-temporary impairment losses	(805)	(749)	(1,099)	(3,462)
Portion of loss recognized in other comprehensive income	(8,326)	(12,760)	(9,190)	(28,748)
Net other-than-temporary credit impairment losses	$ (9,131)	$ (13,509)	$ (10,289)	$ (32,210)

The amount by which the fair value of an investment 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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=109979856&loc=SL6284422-111562

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+ References

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- Definition

Amount before tax of other than temporary impairment loss (OTTI) on investments in available-for-sale and held-to-maturity debt securities, recognized in other comprehensive loss and attributable to parent entity.

+ References

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The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

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