Investment In Securities - Unrealized Gain or Loss Position and Components of Carrying Value of Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Principal Amount	$ 11,324,705	$ 6,891,792
Amortized Cost	11,586,293	7,095,824
Gross Unrealized Loss	(792)	(61,295)
Gross Unrealized Gain	356,265	17,425
Fair Value	11,941,766	7,051,954
Fannie Mae
Debt Securities, Available-for-sale [Line Items]
Principal Amount	8,779,331	5,200,450
Amortized Cost	8,975,140	5,337,466
Gross Unrealized Loss	(291)	(46,758)
Gross Unrealized Gain	294,937	16,760
Fair Value	9,269,786	5,307,468
Freddie Mac
Debt Securities, Available-for-sale [Line Items]
Principal Amount	2,522,870	1,362,771
Amortized Cost	2,587,512	1,417,313
Gross Unrealized Loss	(40)	(13,815)
Gross Unrealized Gain	61,323	657
Fair Value	2,648,795	1,404,155
Ginnie Mae
Debt Securities, Available-for-sale [Line Items]
Principal Amount	22,504	328,571
Amortized Cost	23,641	341,045
Gross Unrealized Loss	(461)	(722)
Gross Unrealized Gain	5	8
Fair Value	$ 23,185	$ 340,331

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).

+ References

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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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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