Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015 Grade	Dec. 31, 2014 Grade	Mar. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,908,535	$ 5,767,743	$ 5,106,726
Loans	4,074,425	4,002,097	3,496,139
Loans	685,629	629,988
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	6.07	5.45
Loans	1,819,049	1,773,945
Commercial and industrial, Other commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	6.22	6.16
Loans	4,089,486	3,993,798
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	6.82	6.76
Loans	3,388,796	3,372,109
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	7.13	6.97
Loans	685,629	629,988
Risk grades 1-8 [Member] \| Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	5.88	5.37
Loans	1,734,701	1,740,455
Risk grades 1-8 [Member] \| Commercial and industrial, Other commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	5.97	5.93
Loans	3,848,801	3,785,171
Risk grades 1-8 [Member] \| Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	6.57	6.53
Loans	3,135,623	3,148,339
Risk grades 1-8 [Member] \| Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	6.98	6.91
Loans	639,505	617,805
Risk grade 9 [Member] \| Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	9.00	9.00
Loans	24,922	27,313
Risk grade 9 [Member] \| Commercial and industrial, Other commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	9.00	9.00
Loans	73,842	65,166
Risk grade 9 [Member] \| Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	9.00	9.00
Loans	104,344	72,906
Risk grade 9 [Member] \| Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	9.00	9.00
Loans	41,422	8,003
Risk grade 10 [Member] \| Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	10.00	10.00
Loans	36,632	161
Risk grade 10 [Member] \| Commercial and industrial, Other commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	10.00	10.00
Loans	87,050	54,519
Risk grade 10 [Member] \| Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	10.00	10.00
Loans	90,872	87,889
Risk grade 10 [Member] \| Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	10.00	10.00
Loans	1,900	1,323
Risk grade 11 [Member] \| Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	11.00	11.00
Loans	22,158	5,380
Risk grade 11 [Member] \| Commercial and industrial, Other commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	11.00	11.00
Loans	46,828	55,034
Risk grade 11 [Member] \| Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	11.00	11.00
Loans	40,812	43,336
Risk grade 11 [Member] \| Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	11.00	11.00
Loans	62	64
Risk grade 12 [Member] \| Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	12.00	12.00
Loans	636	636
Risk grade 12 [Member] \| Commercial and industrial, Other commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	12.00	12.00
Loans	30,980	31,683
Risk grade 12 [Member] \| Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	12.00	12.00
Loans	17,074	19,501
Risk grade 12 [Member] \| Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	12.00	12.00
Loans	2,740	2,793
Risk grade 13 [Member] \| Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	13.00	13.00
Loans	0	0
Risk grade 13 [Member] \| Commercial and industrial, Other commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	13.00	13.00
Loans	1,985	2,225
Risk grade 13 [Member] \| Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	13.00	13.00
Loans	71	138
Risk grade 13 [Member] \| Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	13.00	13.00
Loans	$ 0	$ 0

Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) (USD $)
In Thousands, unless otherwise specified

3 Months Ended

12 Months Ended

Mar. 31, 2015

Grade

Dec. 31, 2014

Grade

Mar. 31, 2014

Accounts, Notes, Loans and Financing Receivable [Line Items]

Loans

$ 5,908,535

$ 5,767,743