Fair Value - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Detail) - Commercial mortgages - Fair Value Measurement on Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Impaired loans:
Impaired loans – commercial mortgages	$ 324	$ 11
Quoted Prices in Active Markets for Identical Assets (Level 1)
Impaired loans:
Impaired loans – commercial mortgages	0	0
Significant Other Observable Inputs (Level 2)
Impaired loans:
Impaired loans – commercial mortgages	0	0
Significant Unobservable Inputs (Level 3)
Impaired loans:
Impaired loans – commercial mortgages	$ 324	$ 11