Loans and Leases (Allowance For Loan And Lease Losses By Portfolio Segment) (Detail) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015		Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance, beginning of period					$ 174,990					$ 159,264	$ 174,990		$ 159,264		$ 152,573
Provision (benefit) charged to expense	$ 12,500		$ 14,250	$ 14,000	15,600	$ 13,800		$ 13,000	$ 12,750	9,750	56,350		49,300		37,250
Losses charged off											(46,912)		(43,560)		(44,426)
Recoveries											9,892		9,986		13,867
Balance, end of period	194,320					174,990					194,320		174,990		159,264
Individually evaluated for impairment	18,593					22,204					18,593		22,204		25,301
Collectively evaluated for impairment	175,727					152,786					175,727		152,786		133,963
Individually evaluated for impairment	249,355					279,171					249,355		279,171		330,856
Collectively evaluated for impairment	16,777,233					15,392,564					16,777,233		15,392,564		13,569,169
Total Loans and Leases	17,026,588	[1],[2]				15,671,735	[1],[2]				17,026,588	[1],[2]	15,671,735	[1],[2]	13,900,025
Residential Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance, beginning of period					25,876					25,452	25,876		25,452		23,027
Provision (benefit) charged to expense											230		6,057		7,315
Losses charged off											(4,636)		(6,508)		(6,214)
Recoveries											1,756		875		1,324
Balance, end of period	23,226					25,876					23,226		25,876		25,452
Individually evaluated for impairment	8,090					10,364					8,090		10,364		12,094
Collectively evaluated for impairment	15,136					15,512					15,136		15,512		13,358
Individually evaluated for impairment	119,424					134,448					119,424		134,448		141,982
Collectively evaluated for impairment	4,135,258					3,926,553					4,135,258		3,926,553		3,367,193
Total Loans and Leases	4,254,682					4,061,001					4,254,682		4,061,001		3,509,175
Consumer Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance, beginning of period					42,052					43,518	42,052		43,518		41,951
Provision (benefit) charged to expense											18,507		11,847		17,224
Losses charged off											(20,669)		(17,679)		(20,712)
Recoveries											5,343		4,366		5,055
Balance, end of period	45,233					42,052					45,233		42,052		43,518
Individually evaluated for impairment	2,903					3,477					2,903		3,477		4,237
Collectively evaluated for impairment	42,330					38,575					42,330		38,575		39,281
Individually evaluated for impairment	45,719					48,425					45,719		48,425		50,249
Collectively evaluated for impairment	2,638,781					2,654,135					2,638,781		2,654,135		2,499,152
Total Loans and Leases	2,684,500					2,702,560					2,684,500		2,702,560		2,549,401
Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance, beginning of period					59,977					47,068	59,977		47,068		46,655
Provision (benefit) charged to expense											28,662		21,693		9,712
Losses charged off											(18,360)		(11,522)		(13,668)
Recoveries											1,626		2,738		4,369
Balance, end of period	71,905					59,977					71,905		59,977		47,068
Individually evaluated for impairment	7,422					5,197					7,422		5,197		2,710
Collectively evaluated for impairment	64,483					54,780					64,483		54,780		44,358
Individually evaluated for impairment	53,037					56,581					53,037		56,581		36,176
Collectively evaluated for impairment	4,887,894					4,259,418					4,887,894		4,259,418		3,713,094
Total Loans and Leases	4,940,931					4,315,999					4,940,931		4,315,999		3,749,270
Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance, beginning of period					41,598					37,148	41,598		37,148		36,754
Provision (benefit) charged to expense											7,930		11,381		2,746
Losses charged off											(2,682)		(7,578)		(3,237)
Recoveries											631		647		885
Balance, end of period	47,477					41,598					47,477		41,598		37,148
Individually evaluated for impairment	169					3,163					169		3,163		6,232
Collectively evaluated for impairment	47,308					38,435					47,308		38,435		30,916
Individually evaluated for impairment	24,755					39,295					24,755		39,295		101,817
Collectively evaluated for impairment	4,486,091					3,952,354					4,486,091		3,952,354		3,452,611
Total Loans and Leases	4,510,846					3,991,649					4,510,846		3,991,649		3,554,428
Finance Leases Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance, beginning of period					$ 5,487					$ 6,078	5,487		6,078		4,186
Provision (benefit) charged to expense											1,021		(1,678)		253
Losses charged off											(565)		(273)		(595)
Recoveries											536		1,360		2,234
Balance, end of period	6,479					5,487					6,479		5,487		6,078
Individually evaluated for impairment	9					3					9		3		28
Collectively evaluated for impairment	6,470					5,484					6,470		5,484		6,050
Individually evaluated for impairment	6,420					422					6,420		422		632
Collectively evaluated for impairment	629,209					600,104					629,209		600,104		537,119
Total Loans and Leases	$ 635,629					$ 600,526					$ 635,629		$ 600,526		$ 537,751

[1] At December 31, 2016, the Company had pledged $6.4 billion of eligible loans as collateral to support borrowing capacity at the FHLB of Boston and the FRB of Boston.

[2] Loans and leases include net deferred fees and net premiums and discounts of $17.3 million and $18.0 million at December 31, 2016 and December 31, 2015, respectively.