Goodwill and Other Intangible Assets - Narrative (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2018	Mar. 23, 2018	Jan. 12, 2018	Dec. 31, 2017	Jul. 31, 2017
Goodwill
Goodwill acquired during the year (preliminary allocation) and adjustments			$ 34,870
Non-compete agreement					$ 156
Title plant assets	$ 6,800	$ 6,800	6,800			$ 6,700
Gibraltar Private Bank & Trust Company [Member]
Goodwill
Goodwill acquired during the year (preliminary allocation) and adjustments	10,000	$ 52,500
Core deposit intangible assets				$ 18,529
Sabadell United
Goodwill
Goodwill acquired during the year (preliminary allocation) and adjustments	$ 22,400		$ 21,000
Core deposit intangible assets							$ 66,600