Loans and Leases - Schedule of Carrying Amount of Loans Acquired (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Mar. 23, 2018	Dec. 31, 2017	Jul. 31, 2017	Jun. 30, 2017
Accounts, Notes, Loans and Financing Receivable
Expected losses and foregone interest	$ (151,009)					$ (156,687)
Loans acquired with no evidence of deteriorated credit quality	18,274			$ 14,953
Related allowance	(17,587)			(17,352)
Sabadell United
Accounts, Notes, Loans and Financing Receivable
Loans acquired					$ 4,030,777
Covered loans acquired	143,800			$ 158,600
Gibraltar Private Bank & Trust Company [Member]
Accounts, Notes, Loans and Financing Receivable
Loans acquired		$ 1,500,000	$ 1,465,278
Contractually required principal and interest at acquisition	1,695,918
Expected losses and foregone interest	(19,952)
Cash flows expected to be collected at acquisition	1,675,966
Loans acquired with no evidence of deteriorated credit quality	1,455,044
Impaired Financing Receivable, with Related Allowance, Recorded Investment	43,779
Related allowance	(31,174)
Impaired Financing Receivable, Expected Cash Flow	12,605
Accretable yield	(2,371)
Loans acquired with deteriorated credit quality	$ 10,234