Debt and Equity Securities - Maturity of Amortized Cost and Estimated Market Value of Debt Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Held-to-Maturity
Held-to-maturity, Due in one year or less, Amortized Cost	$ 0
Held-to-maturity, Due after one year through five years, Amortized Cost	0
Held-to-maturity, Due after five years through ten years, Amortized Cost	0
Held-to-maturity, Due after ten years, Amortized Cost	0
Held-to-maturity, Gross Amortized Cost	0
Held-to-maturity, Due in one year or less, Estimated Market Value	0
Held-to-maturity, Due after one year through five years, Estimated Market Value	0
Held-to-maturity, Due after five years through ten years, Estimated Market Value	0
Held-to-maturity, Due after ten years, Estimated Market Value	0
Held-to-maturity, Gross Estimated Market Value	0
Available-For-Sale
Available-for-sale securities, Due in one year or less, Amortized Cost	2,036
Available-for-sale securities, Due after one year through five years, Amortized Cost	30,486
Available-for-sale securities, Due after five years through ten years, Amortized Cost	130,510
Available-for-sale securities, Due after ten years, Amortized Cost	139,692
Amortized Cost	302,724	$ 338,449
Available-for-sale securities, Due in one year or less, Estimated Market Value	2,036
Available-for-sale securities, Due after one year through five years, Estimated Market Value	29,430
Available-for-sale securities, Due after five years through ten years, Estimated Market Value	123,075
Available-for-sale securities, Due after ten years, Estimated Market Value	132,983
Available-for-sale securities, Estimated Market Value	$ 287,524