Loans - Unallocated Portion of Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 149,558	$ 153,056	$ 153,045	$ 149,714	$ 161,880	$ 135,859
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2,130		9,186
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	147,428		143,859
Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	86,802		91,663
Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,130		9,186
General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	21,245		16,843
Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	39,381		35,353
Commercial Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	48,906	45,583	52,915	47,578	45,084	42,993
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,780		5,436
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	47,126		47,479
Commercial Portfolio Segment [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	28,504		33,251
Commercial Portfolio Segment [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,780		5,436
Commercial Portfolio Segment [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	8,418		6,708
Commercial Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	10,204		7,520
Commercial and industrial, Energy [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	54,277	61,793	60,653	66,339	84,973	54,696
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	350		3,750
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	53,927		56,903
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	32,452		34,626
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	350		3,750
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	5,934		3,769
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	15,541		18,508
Commercial Real Estate Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	33,002	34,009	30,213	27,063	23,587	24,313
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	33,002		30,213
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	17,975		16,976
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	0		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	4,724		5,004
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	10,303		8,233
Consumer Real Estate [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	5,535	4,823	4,238	3,935	3,786	4,659
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	5,535		4,238
Consumer Real Estate [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,416		2,225
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	2,032		1,506
Consumer Real Estate [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,087		507
Consumer and Other [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	7,838	$ 6,848	5,026	$ 4,799	$ 4,450	$ 9,198
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	7,838		5,026
Consumer and Other [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	5,455		4,585
Consumer and Other [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		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	137		(144)
Consumer and Other [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 2,246		$ 585