Past Due Loans, Allowance For Credit Losses, Impaired Loans, and TDRs (Detail) - Aging Analysis of Past Due Loans and Non-Performing Loans by Class - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 7,161,715	$ 6,736,853
Commercial And Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,516,197	2,230,947
Total Past Due	8,529	22,609
Total loans	2,524,726	2,253,556
Non-accrual Loans	5,587	22,693
90 Days Past Due Loans, Still Accruing Interest	857	205
Agricultural
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	387,109	355,982
Total Past Due	331	2,267
Total loans	387,440	358,249
Non-accrual Loans	355	360
90 Days Past Due Loans, Still Accruing Interest	0	0
Office, Retail, and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,386,383	1,463,724
Total Past Due	9,071	14,655
Total loans	1,395,454	1,478,379
Non-accrual Loans	6,875	12,939
90 Days Past Due Loans, Still Accruing Interest	4	76
Multi-family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	526,625	562,625
Total Past Due	1,699	1,796
Total loans	528,324	564,421
Non-accrual Loans	796	754
90 Days Past Due Loans, Still Accruing Interest	548	83
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	216,377	197,255
Total Past Due	505	6,981
Total loans	216,882	204,236
Non-accrual Loans	905	6,981
90 Days Past Due Loans, Still Accruing Interest	0	0
Other Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	922,531	876,609
Total Past Due	8,659	11,288
Total loans	931,190	887,897
Non-accrual Loans	5,611	6,970
90 Days Past Due Loans, Still Accruing Interest	661	438
Total Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,051,916	3,100,213
Total Past Due	19,934	34,720
Total loans	3,071,850	3,134,933
Non-accrual Loans	14,187	27,644
90 Days Past Due Loans, Still Accruing Interest	1,213	597
Total Corporate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	5,955,222	5,687,142
Total Past Due	28,794	59,596
Total loans	5,984,016	5,746,738
Non-accrual Loans	20,129	50,697
90 Days Past Due Loans, Still Accruing Interest	2,070	802
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	647,175	535,587
Total Past Due	6,293	7,598
Total loans	653,468	543,185
Non-accrual Loans	5,310	6,290
90 Days Past Due Loans, Still Accruing Interest	216	145
1-4 family mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	350,980	287,892
Total Past Due	4,874	3,571
Total loans	355,854	291,463
Non-accrual Loans	3,416	2,941
90 Days Past Due Loans, Still Accruing Interest	528	166
Installment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	136,780	75,428
Total Past Due	822	604
Total loans	137,602	76,032
Non-accrual Loans	20	43
90 Days Past Due Loans, Still Accruing Interest	69	60
Total consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,134,935	898,907
Total Past Due	11,989	11,773
Total loans	1,146,924	910,680
Non-accrual Loans	8,746	9,274
90 Days Past Due Loans, Still Accruing Interest	813	371
Total Loans Excluding Covered Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	7,090,157	6,586,049
Total Past Due	40,783	71,369
Total loans	7,130,940	6,657,418
Non-accrual Loans	28,875	59,971
90 Days Past Due Loans, Still Accruing Interest	2,883	1,173
Covered Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	29,808	66,331
Total Past Due	967	13,104
Total loans	30,775	79,435
Non-accrual Loans	555	6,186
90 Days Past Due Loans, Still Accruing Interest	174	5,002
Loans Total
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	7,119,965	6,652,380
Total Past Due	41,750	84,473
Total loans	7,161,715	6,736,853
Non-accrual Loans	29,430	66,157
90 Days Past Due Loans, Still Accruing Interest	3,057	6,175
30-89 Days Past Due | Commercial And Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,956	19,505
30-89 Days Past Due | Agricultural
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	245	1,934
30-89 Days Past Due | Office, Retail, and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,647	2,340
30-89 Days Past Due | Multi-family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	541	1,261
30-89 Days Past Due | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
30-89 Days Past Due | Other Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,575	5,412
30-89 Days Past Due | Total Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,763	9,013
30-89 Days Past Due | Total Corporate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	11,964	30,452
30-89 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,247	3,216
30-89 Days Past Due | 1-4 family mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,680	2,246
30-89 Days Past Due | Installment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	753	506
30-89 Days Past Due | Total consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,680	5,968
30-89 Days Past Due | Total Loans Excluding Covered Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	18,644	36,420
30-89 Days Past Due | Covered Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	405	2,714
30-89 Days Past Due | Loans Total
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	19,049	39,134
90 Days or More Past Due | Commercial And Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,573	3,104
90 Days or More Past Due | Agricultural
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	86	333
90 Days or More Past Due | Office, Retail, and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,424	12,315
90 Days or More Past Due | Multi-family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,158	535
90 Days or More Past Due | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	505	6,981
90 Days or More Past Due | Other Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,084	5,876
90 Days or More Past Due | Total Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	13,171	25,707
90 Days or More Past Due | Total Corporate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	16,830	29,144
90 Days or More Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,046	4,382
90 Days or More Past Due | 1-4 family mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,194	1,325
90 Days or More Past Due | Installment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	69	98
90 Days or More Past Due | Total consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,309	5,805
90 Days or More Past Due | Total Loans Excluding Covered Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	22,139	34,949
90 Days or More Past Due | Covered Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	562	10,390
90 Days or More Past Due | Loans Total
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 22,701	$ 45,339