Loans, Leases and Allowance for Credit Losses - Schedule of Held for Investment Loan Portfolio Composition of Loans, Leases and Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit Mark on Acquired Loans	$ 19,700	$ 27,000
Net deferred loan fees and costs	(32,100)	(25,300)
Receivable with Imputed Interest, Discount	7,900	8,500
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield		9,324	$ 15,177
Loans, net of deferred loan fees and costs	16,138,260	15,093,935
Allowance for credit losses	(147,083)	(140,050)
Loans, net	15,991,177	14,953,885
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	7,278,408	6,841,381
Commercial Real Estate Non Owner Occupied Multi Family [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	4,010,576	3,904,011
Owner occupied [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	2,270,531	2,241,613
Allowance for credit losses	(14,651)	(13,884)
Construction and land development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	1,978,287	1,632,204
Allowance for credit losses	(22,159)	(19,599)
Residential real estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	545,256	425,940
Allowance for credit losses	(6,849)	(5,500)
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, net of deferred loan fees and costs	55,202	48,786
Allowance for credit losses	$ (852)	$ (776)