Past Due Loans, Allowance For Credit Losses, Impaired Loans, and TDRs (Detail) - Credit Quality Indicators by Class, Excluding Covered Loans - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Commercial And Industrial | Pass
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		$ 2,379,992	$ 2,115,170
Commercial And Industrial | Special Mention
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[1],[2]	86,263	84,615
Commercial And Industrial | Substandard
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[2],[3]	52,884	31,078
Commercial And Industrial | Non Accrual
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[4]	5,587	22,693
Commercial And Industrial | Total
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		2,524,726	2,253,556
Agricultural | Pass
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		381,523	357,595
Agricultural | Special Mention
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[1],[2]	0	294
Agricultural | Substandard
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[2],[3]	5,562	0
Agricultural | Non Accrual
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[4]	355	360
Agricultural | Total
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		387,440	358,249
Office, Retail, and Industrial | Pass
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		1,320,164	1,393,885
Office, Retail, and Industrial | Special Mention
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[1],[2]	32,627	38,891
Office, Retail, and Industrial | Substandard
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[2],[3]	35,788	32,664
Office, Retail, and Industrial | Non Accrual
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[4]	6,875	12,939
Office, Retail, and Industrial | Total
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		1,395,454	1,478,379
Multi-family | Pass
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		517,412	553,255
Multi-family | Special Mention
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[1],[2]	6,146	6,363
Multi-family | Substandard
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[2],[3]	3,970	4,049
Multi-family | Non Accrual
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[4]	796	754
Multi-family | Total
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		528,324	564,421
Construction | Pass
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		201,496	178,992
Construction | Special Mention
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[1],[2]	4,678	5,776
Construction | Substandard
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[2],[3]	9,803	12,487
Construction | Non Accrual
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[4]	905	6,981
Construction | Total
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		216,882	204,236
Other Commercial Real Estate | Pass
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		898,746	829,003
Other Commercial Real Estate | Special Mention
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[1],[2]	13,179	32,517
Other Commercial Real Estate | Substandard
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[2],[3]	13,654	19,407
Other Commercial Real Estate | Non Accrual
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[4]	5,611	6,970
Other Commercial Real Estate | Total
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		931,190	887,897
Total Commercial Real Estate | Pass
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		2,937,818	2,955,135
Total Commercial Real Estate | Special Mention
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[1],[2]	56,630	83,547
Total Commercial Real Estate | Substandard
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[2],[3]	63,215	68,607
Total Commercial Real Estate | Non Accrual
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[4]	14,187	27,644
Total Commercial Real Estate | Total
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		3,071,850	3,134,933
Total Corporate Loans | Pass
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		5,699,333	5,427,900
Total Corporate Loans | Special Mention
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[1],[2]	142,893	168,456
Total Corporate Loans | Substandard
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[2],[3]	121,661	99,685
Total Corporate Loans | Non Accrual
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class	[4]	20,129	50,697
Total Corporate Loans | Total
Credit Quality Indicators by Class, Excluding Covered Loans [Line Items]
Credit Quality Indicators by Class		$ 5,984,016	$ 5,746,738

[1] Loans categorized as special mention exhibit potential weaknesses that require the close attention of management since these potential weaknesses may result in the deterioration of repayment prospects in the future.

[2] Total special mention and substandard loans includes accruing TDRs of $862,000 as of December 31, 2015 and $1.8 million as of December 31, 2014.

[3] Loans categorized as substandard exhibit a well-defined weakness or weaknesses that may jeopardize the liquidation of the debt. These loans continue to accrue interest because they are well secured and collection of principal and interest is expected within a reasonable time.

[4] Loans categorized as non-accrual exhibit a well-defined weakness or weaknesses that may jeopardize the liquidation of the debt or result in a loss if the deficiencies are not corrected.