Debt Securities Available for Sale - Fair value of the debt securities of issuers (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investment
Amortized Cost	$ 15,259,832	$ 17,542,125
Debt Securities Available for Sale, at Fair Value	15,813,301	17,648,473
FNMA
Investment
Amortized Cost	2,934,302	3,113,373
Debt Securities Available for Sale, at Fair Value	3,025,419	3,129,538
Freddie Mac
Investment
Amortized Cost	1,536,397	1,623,116
Debt Securities Available for Sale, at Fair Value	$ 1,588,763	$ 1,638,796

Amount of 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 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant