Fair Value - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Detail) - Fair Value Measurement on Recurring Basis - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Commercial mortgages
Impaired loans:
Impaired loans – commercial mortgages	$ 321	$ 11
Commercial mortgages | Quoted Prices in Active Markets for Identical Assets (Level 1)
Impaired loans:
Impaired loans – commercial mortgages	0	0
Commercial mortgages | Significant Other Observable Inputs (Level 2)
Impaired loans:
Impaired loans – commercial mortgages	0	0
Commercial mortgages | Significant Unobservable Inputs (Level 3)
Impaired loans:
Impaired loans – commercial mortgages	321	$ 11
Commercial, Industrial, and Agricultural
Impaired loans:
Impaired loans – commercial mortgages	3,004
Commercial, Industrial, and Agricultural | Quoted Prices in Active Markets for Identical Assets (Level 1)
Impaired loans:
Impaired loans – commercial mortgages	0
Commercial, Industrial, and Agricultural | Significant Other Observable Inputs (Level 2)
Impaired loans:
Impaired loans – commercial mortgages	0
Commercial, Industrial, and Agricultural | Significant Unobservable Inputs (Level 3)
Impaired loans:
Impaired loans – commercial mortgages	$ 3,004