Mortgage-Backed Securities- Schedule of Available-for-sale Securities Reconciliation (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2018
Debt Disclosure [Abstract]
LIBOR rate	2.50

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

No definition available.

Name:	us-gaap_DebtInstrumentDescriptionOfVariableRateBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration