Debt and Equity Securities - Maturity of Amortized Cost and Estimated Market Value of Debt Securities (Detail) $ in Thousands	Sep. 30, 2015 USD ($)
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities, Due in one year or less, Amortized Cost	$ 2,573
Available-for-sale securities, Due after one year through five years, Amortized Cost	9,955
Available-for-sale securities, Due after five years through ten years, Amortized Cost	3,372
Available-for-sale securities, Due after ten years, Amortized Cost	9,890
Available-for-sale securities, Gross Amortized Cost	25,790
Available-for-sale securities, Due in one year or less, Estimated Market Value	2,592
Available-for-sale securities, Due after one year through five years, Estimated Market Value	10,093
Available-for-sale securities, Due after five years through ten years, Estimated Market Value	3,432
Available-for-sale securities, Due after ten years, Estimated Market Value	9,841
Available-for-sale securities, Gross Estimated Market Value	25,958
Held-to-maturity, Due in one year or less, Amortized Cost	0
Held-to-maturity, Due after one year through five years, Amortized Cost	44,258
Held-to-maturity, Due after five years through ten years, Amortized Cost	120,979
Held-to-maturity, Due after ten years, Amortized Cost	127,376
Held-to-maturity, Gross Amortized Cost	292,613
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	44,487
Held-to-maturity, Due after five years through ten years, Estimated Market Value	121,173
Held-to-maturity, Due after ten years, Estimated Market Value	127,391
Held-to-maturity, Gross Estimated Market Value	$ 293,051