Derivative Instruments - Summary of Gross Principal Notional Outstanding for CDS Contracts (Detail) $ in Thousands	Jun. 30, 2019 USD ($)
Other Derivatives [Member] \| AA Rating [Member]
Ambac Rating
Notional outstanding	$ 284,350

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant