Investment Securities - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) security
Carrying value of pledged securities | $	$ 834.9	$ 928.8
Number of securities in security portfolio		361
Securities in an unrealized loss position		204
Securities in an unrealized loss position for less than 12 months		139
ASU 2016-01
Equity securities, unrealized gain | $	$ 1.6