Past Due Loans, Allowances For Credit Losses, Impaired Loans, and TDRs (Detail) - Impaired Loans Individually Evaluated by Class (Continued) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Average Recorded Balance	$ 48,163		$ 65,916		$ 151,821
Interest income recognized	759	[1]	262	[1]	135	[1]
Commercial And Industrial
Financing Receivable, Impaired [Line Items]
Average Recorded Balance	16,137		20,925		45,101
Interest income recognized	371	[1]	205	[1]	94	[1]
Agricultural
Financing Receivable, Impaired [Line Items]
Average Recorded Balance	0		0		1,138
Interest income recognized	0	[1]	0	[1]	0	[1]
Office Retail And Industrial
Financing Receivable, Impaired [Line Items]
Average Recorded Balance	19,003		24,802		32,439
Interest income recognized	245	[1]	18	[1]	2	[1]
Multi-Family
Financing Receivable, Impaired [Line Items]
Average Recorded Balance	1,245		1,116		6,226
Interest income recognized	5	[1]	8	[1]	0	[1]
Construction
Financing Receivable, Impaired [Line Items]
Average Recorded Balance	5,764		5,932		31,202
Interest income recognized	0	[1]	0	[1]	1	[1]
Other Commercial Real Estate
Financing Receivable, Impaired [Line Items]
Average Recorded Balance	6,014		13,141		35,715
Interest income recognized	138	[1]	31	[1]	38	[1]
Total Commercial Real Estate
Financing Receivable, Impaired [Line Items]
Average Recorded Balance	32,026		44,991		105,582
Interest income recognized	$ 388	[1]	$ 57	[1]	$ 41	[1]

[1] Recorded using the cash basis of accounting.