Fair Value - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Detail) - Commercial Mortgages [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	$ 60	$ 11
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	0	0
Significant Other Observable Inputs (Level 2) [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	$ 60	$ 11