Mortgage-Backed Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Sale of securities	$ 903,822	$ 589,073
Unrealized gain (loss) on trading investments	(14,900)	8,900
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Sale of securities	903,800	583,200
Gross realized losses on sales of securities	8,800	19,300
Gross realized gains on sales of securities	2,600
Unrealized gain (loss) on trading investments	14,906	(8,890)
Non-Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Sale of securities	20,000	5,800
Gross realized losses on sales of securities		$ 42
Gross realized gains on sales of securities	22
Trading Securities \| Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on trading investments	$ (1,000)		$ (15,900)

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 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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