Repurchase Agreements - Additional Information (Detail)	Sep. 30, 2019 USD ($)
Debt Instrument [Line Items]
Weighted Average Interest Rate	3.20%
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	1.20%
Barclays MRA [Member]
Debt Instrument [Line Items]
Maximum facility size	$ 750,000,000

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant