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).

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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).

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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).

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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.

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Mortgage-Backed Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Sale of securities	$ 1,335,932		$ 2,239,754	$ 589,073
Unrealized gain (loss) on trading investments	1,000	$ 2,700	15,900	(11,600)
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Sale of securities	1,340,000		2,240,000	583,200
Gross realized losses on sales of securities	4,300		20,500	19,300
Gross realized gains on sales of securities			7,700
Gross unrealized gains (losses) on sales of securities	5,000
Unrealized gain (loss) on trading investments	989	$ (2,677)	15,895	(11,567)
Non-Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Sale of securities	$ 0		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					$ (15,900)