Debt Securities Available for Sale - Fair value of the debt securities of issuers (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment
Amortized Cost	$ 16,351,743	$ 13,478,562
FNMA
Investment
Amortized Cost	3,334,102	2,999,110
Fair Value	3,351,007	2,901,904
Freddie Mac
Investment
Amortized Cost	1,722,954	1,095,855
Fair Value	$ 1,739,165	$ 1,058,013

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563

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