Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Unrealized gains (losses) on debt securities available for sale:
Beginning balance, before tax, unrealized gains (losses) on debt securities available for sale	$ 14,402	$ (6,504)
Beginning balance, tax, unrealized gains (losses) on debt securities available for sale	(5,509)	2,509
Beginning balance, net of tax, unrealized gains (losses) on debt securities available for sale	(5,127)	6,741	$ (5,127)	$ 8,893	$ 6,741	$ (3,995)
Net unrealized holdings gain (loss) arising during the period, before tax	(19,242)	17,505
Net unrealized holdings gain (loss) arising during the period, tax effect	4,801	(6,719)
Net unrealized holdings gain (loss) arising during the period, net of tax	(14,441)	10,786
Amounts reclassified to (gain) loss on investment securities, before tax	(405)	(82)
Amounts reclassified to (gain) loss on investment securities, tax effect	101	32
Amounts reclassified to (gain) loss on investment securities, net of tax	(304)	(50)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities, Cumulative Effect Of Adoption Of Accounting Standards, Before Tax	(1,587)	0
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities, Cumulative Effect Of Adoption Of Accounting Standards, Tax	2,312	0
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities, Cumulative Effect Of Adoption Of Accounting Standards	725	0
Ending balance, before tax, unrealized gains (losses) on debt securities available for sale	(6,832)	10,919
Ending balance, tax, unrealized gains (losses) on debt securities available for sale	1,705	(4,178)
Ending balance, net of tax, unrealized gains (losses) on debt securities available for sale	(5,127)	6,741
Unrealized gains (losses) on derivative instruments:
Beginning balance, before tax, unrealized gains (losses) on derivative instruments			(1,483)	0	0	0
Beginning balance, tax, unrealized gains (losses) on derivative instruments			370	0	0	0
Beginning balance, net of tax, unrealized gains (losses) on derivative instruments			$ (1,113)	$ 0	$ 0	$ 0
Net unrealized holdings gain (loss) arising during the period, before tax	(1,483)	0
Net unrealized holdings gain (loss) arising during the period, tax effect	370	0
Net unrealized holdings gain (loss) arising during the period, net of tax	(1,113)	0
Amounts reclassified to (gain) loss on investment securities, before tax	0	0
Amounts reclassified to (gain) loss on investment securities, tax effect	0	0
Amounts reclassified to (gain) loss on investment securities, net of tax effect	0	0
Balance at end of period, before tax	(8,315)	10,919
Balance at end of period, tax effect	2,075	(4,178)
Balance at end of period, net of tax	$ (6,240)	$ 6,741