Restricted Investments - Restricted Investment Balances (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	May 04, 2017
Schedule Of Investments [Line Items]
Restricted investments	$ 4,622	$ 94,467	$ 156,443
BOEM platform abandonment [Member]
Schedule Of Investments [Line Items]
Restricted investments	313	90,162
SPBPC Collateral Contractual pipeline and surface facilities abandonment [Member]
Schedule Of Investments [Line Items]
Restricted investments	$ 4,309	$ 4,305