Mortgage-Backed Securities - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Sale of securities	$ 2,950,885	$ 787,260	$ 736,900
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Sale of securities	2,950,000	781,000
Gross realized losses on sales of securities	21,700	24,200
Gross realized gains on sales of securities	11,800	600
Unrealized gain (loss) on trading investments	17,036	(4,911)	$ 2,793
Non-Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Sale of securities		6,000
Gross realized losses on sales of securities		42
Gross realized gains on sales of securities		217
Trading Securities
Schedule Of Available For Sale Securities [Line Items]
Gross unrealized gains (losses) on sales of securities	17,000	$ (4,900)
Non-Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Sale of securities	30,000
Gross realized gains on sales of securities	$ 310

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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Amount of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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Amount of cash inflow from sale of investment in mortgage-backed security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

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