Allowance For Loan Losses and Credit Quality Information (Nonaccrual and Past Due Loans) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing	$ 7,392		$ 13,577
60-89 Days Past Due and Still Accruing	1,445		2,253
90 Days Past Due and Still Accruing	1,869		965
Due And Still Accruing	10,706		16,795
Accruing Current Balances	2,651,113		2,667,781
Nonaccrual Loans	39,940		71,093
Total Loans	2,701,759		2,755,669
Financing Receivable Recorded Investment Thirty To Fifty Nine Days Past Due Still Accruing Percentage Of Total Loans	0.28%		0.49%
Financing Receivable Recorded Investment Sixty To Eighty Nine Days Past Due Still Accruing Percentage Of Total Loans	0.05%		0.08%
Financing Receivable Recorded Investment Greater Than Ninety Days Past Due Still Accruing Percentage Of Total Loans	0.07%		0.04%
Financing Receivable Recorded Investment Past Due Still Accruing Percentage Of Total Loans	0.40%		0.61%
Financing Receivable Recorded Investment Current Percentage Of Total Loans	98.12%		96.81%
Financing Receivable, Recorded Investment, Nonaccrual Status Percentage	1.48%		2.58%
Commercial Loans	100.00%		100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing	332		1,087
60-89 Days Past Due and Still Accruing			63
90 Days Past Due and Still Accruing	1,869		78
Due And Still Accruing	2,201		1,228
Accruing Current Balances	692,425		1,435,876
Nonaccrual Loans	3,579		23,080
Total Loans	698,205		1,460,184
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing			479
60-89 Days Past Due and Still Accruing	80		243
Due And Still Accruing	80		722
Accruing Current Balances	598,601		605,764
Nonaccrual Loans	5,875		15,814
Total Loans	604,556		622,300
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing			3,727
Due And Still Accruing			3,727
Accruing Current Balances	111,557		80,074
Nonaccrual Loans	2,620		22,124
Total Loans	114,177		105,925
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing	5,272		5,501
60-89 Days Past Due and Still Accruing	1,218		1,238
90 Days Past Due and Still Accruing			887
Due And Still Accruing	6,490		7,626
Accruing Current Balances	234,498		258,820
Nonaccrual Loans	9,354		9,057
Total Loans	250,342		275,503
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing	1,788		2,783
60-89 Days Past Due and Still Accruing	147		709
Due And Still Accruing	1,935		3,492
Accruing Current Balances	276,362		287,247
Nonaccrual Loans	5,188		1,018
Total Loans	283,485		291,757
Owner Occupied Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accruing Current Balances	737,670	[1]
Nonaccrual Loans	13,324	[1]
Total Loans	$ 750,994	[1]

[1] Prior to 2012, owner-occupied commercial loans were included in commercial loan balances.