Loans - Unallocated Portion of Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 137,578	$ 150,226	$ 155,364	$ 154,303	$ 149,558	$ 153,045
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	21,488		20,820
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	116,090		134,544
Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	65,133		75,481
Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	21,488		20,820
General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	20,903		23,283
Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	30,054		35,780
Commercial Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	50,991	57,713	59,614	48,437	48,906	52,915
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	4,622		7,553
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	46,369		52,061
Commercial Portfolio Segment [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	25,582		26,401
Commercial Portfolio Segment [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	4,622		7,553
Commercial Portfolio Segment [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	9,361		9,112
Commercial Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	11,426		16,548
Commercial and industrial, Energy [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	31,445	37,313	51,528	51,913	54,277	60,653
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	12,672		13,267
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	18,773		38,261
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	9,654		22,073
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	12,672		13,267
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	5,849		7,964
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	3,270		8,224
Commercial Real Estate Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	39,204	38,918	30,948	38,075	33,002	30,213
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2,599		0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	36,605		30,948
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	20,441		18,931
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	2,599		0
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	4,187		4,165
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	11,977		7,852
Consumer Real Estate [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	6,194	6,336	5,657	6,875	5,535	4,238
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	6,194		5,657
Consumer Real Estate [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,641		2,473
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	1,620		2,133
Consumer Real Estate [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,933		1,051
Consumer and Other [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	9,744	$ 9,946	7,617	$ 9,003	$ 7,838	$ 5,026
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,595		0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	8,149		7,617
Consumer and Other [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	6,815		5,603
Consumer and Other [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,595		0
Consumer and Other [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	(114)		(91)
Consumer and Other [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 1,448		$ 2,105