Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Banking_Center	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Number of banking centers | Banking_Center	51
Stranded tax effect			$ 1,700
Excess tax benefit	$ 2,500	$ 9,200
Retained Earnings
Cumulative adjustment from adoption of new accounting standards (1)			(725)
ASU 2016-01
Equity securities, unrealized gain			1,600
ASU 2016-01 | Retained Earnings
Cumulative adjustment from adoption of new accounting standards (1)			$ 1,000