Investment Securities - Investment Ratings of All Privately Issued Mortgage-Backed Securities and Other Debt Securities (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Amortized Cost	$ 304,748
Total securities with disclosed rating, Estimated Fair Value	271,768
A or Better [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	46,313
BBB [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	61,070
BB [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	32,684
B or Less [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	87,233
Not Rated [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	44,468
Obligations of States and Political Subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Amortized Cost	27,117
Total securities with disclosed rating, Estimated Fair Value	27,211
Obligations of States and Political Subdivisions [Member] | A or Better [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	15,874
Obligations of States and Political Subdivisions [Member] | Not Rated [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	11,337
Privately Issued [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Amortized Cost	135,716
Total securities with disclosed rating, Estimated Fair Value	110,715
Privately Issued [Member] | A or Better [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	23,459
Privately Issued [Member] | BBB [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	16
Privately Issued [Member] | B or Less [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	87,233
Privately Issued [Member] | Not Rated [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	7
Other Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Amortized Cost	141,915
Total securities with disclosed rating, Estimated Fair Value	133,842
Other Debt Securities [Member] | A or Better [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	6,980
Other Debt Securities [Member] | BBB [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	61,054
Other Debt Securities [Member] | BB [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	32,684
Other Debt Securities [Member] | Not Rated [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total securities with disclosed rating, Estimated Fair Value	$ 33,124