Investments In Securities - Unrealized Gain or Loss Position and Components of Carrying Value of Available for Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Principal Amount	$ 1,893,769	$ 11,324,705
Amortized Cost	1,952,400	11,586,293
Gross Unrealized Loss	(34)	(792)
Gross Unrealized Gain	200,231	356,265
Fair Value	2,152,597	11,941,766
Fannie Mae
Debt Securities, Available-for-sale [Line Items]
Principal Amount	1,468,314	8,779,331
Amortized Cost	1,510,059	8,975,140
Gross Unrealized Loss	(22)	(291)
Gross Unrealized Gain	181,302	294,937
Fair Value	1,691,339	9,269,786
Freddie Mac
Debt Securities, Available-for-sale [Line Items]
Principal Amount	407,397	2,522,870
Amortized Cost	423,830	2,587,512
Gross Unrealized Loss	0	(40)
Gross Unrealized Gain	18,746	61,323
Fair Value	442,576	2,648,795
Ginnie Mae
Debt Securities, Available-for-sale [Line Items]
Principal Amount	18,058	22,504
Amortized Cost	18,511	23,641
Gross Unrealized Loss	(12)	(461)
Gross Unrealized Gain	183	5
Fair Value	$ 18,682	$ 23,185

Debt Securities, Available-for-sale, Principal Amount

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) 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, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) 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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Amortized cost of 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 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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