Investment Securities - Narrative (Detail) $ in Millions	Jun. 30, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment
Investments, Debt and Equity Securities [Abstract]
Carrying value of securities pledged to secure public deposits and for other purposes as required by law	$ 354.0	$ 356.7
Pledging requirements for credit derivatives and SWAP agreements - securities	$ 1.3	$ 1.4
Number of investments in non-federal issuer representing more than 10% of shareholders' equity | Investment	0	0
Maximum investment in any single non-federal issuer representing shareholders' equity	10.00%	10.00%