Insurance Activity - Components of Liability for Losses and Loss Adjustment Reserves (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Effects of Reinsurance [Line Items]
Total losses and loss adjustment reserves	$ 408	$ 474	[1]	$ 811
Case-basis reserves [Member]
Effects of Reinsurance [Line Items]
Total losses and loss adjustment reserves	12	151
IBNR reserves [Member]
Effects of Reinsurance [Line Items]
Total losses and loss adjustment reserves	1	1
Premium deficiency reserves [Member]
Effects of Reinsurance [Line Items]
Total losses and loss adjustment reserves	$ 395	$ 322

[1] Balances derived from audited financial statements as of December 31, 2017.