Loans - Unallocated Portion of Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 136,350	$ 132,132	$ 149,885	$ 155,364
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	19,973	16,235
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	116,377	115,897
Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	71,570	65,398
Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	19,973	16,235
General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	20,452	22,100
Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	24,355	28,399
Commercial Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	58,571	48,580	57,733	59,614
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	6,698	2,558
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	51,873	46,022
Commercial Portfolio Segment [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	31,431	25,351
Commercial Portfolio Segment [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	6,698	2,558
Commercial Portfolio Segment [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	10,870	10,062
Commercial Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	9,572	10,609
Commercial and industrial, Energy [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	25,343	29,052	39,039	51,528
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	8,996	9,671
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	16,347	19,381
Commercial and industrial, Energy [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	9,293	9,697
Commercial and industrial, Energy [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	8,996	9,671
Commercial and industrial, Energy [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	4,200	6,014
Commercial and industrial, Energy [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,854	3,670
Commercial Real Estate Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	36,455	38,777	38,474	30,948
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2,855	2,599
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	33,600	36,178
Commercial Real Estate Portfolio Segment [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	20,576	20,817
Commercial Real Estate Portfolio Segment [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,855	2,599
Commercial Real Estate Portfolio Segment [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	3,812	4,366
Commercial Real Estate Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	9,212	10,995
Consumer Real Estate [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	5,661	6,103	6,349	5,657
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	5,661	6,103
Consumer Real Estate [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,571	2,688
Consumer Real Estate [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0	0
Consumer Real Estate [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,654	1,671
Consumer Real Estate [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,436	1,744
Consumer and Other [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	10,320	9,620	$ 8,290	$ 7,617
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,424	1,407
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	8,896	8,213
Consumer and Other [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	7,699	6,845
Consumer and Other [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,424	1,407
Consumer and Other [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	(84)	(13)
Consumer and Other [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 1,281	$ 1,381