Amortized Cost/Cost, Estimated Fair Value and Gross Unrealized (Losses) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, Amortized Cost/Cost	$ 25,473	$ 0
Equity securities, Estimated Fair Value	28,126
Equity securities, Gross Unrealized Gains (Losses) Recorded in Net Investment Income (Loss)	2,653
Banking securities
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, Amortized Cost/Cost	25,473
Equity securities, Estimated Fair Value	28,126
Equity securities, Gross Unrealized Gains (Losses) Recorded in Net Investment Income (Loss)	$ 2,653