Loans and Leases (Commercial, Commercial Real Estate Loans And Equipment Financing Loans Segregated By Risk Rating Exposure) (Detail) - USD ($) $ in Thousands	Jun. 30, 2017		Dec. 31, 2016		Jun. 30, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	$ 17,273,678	[1],[2]	$ 17,026,588	[1],[2]	$ 16,272,029
Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	5,144,171		4,940,931		4,577,482
Commercial Portfolio Segment [Member] | (1) - (6) Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	4,828,252		4,655,007
Commercial Portfolio Segment [Member] | (7) Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	86,109		56,240
Commercial Portfolio Segment [Member] | (8) Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	207,831		226,603
Commercial Portfolio Segment [Member] | (9) Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	21,979		3,081
Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	4,556,208		4,510,846		4,191,087
Commercial Real Estate Portfolio Segment [Member] | (1) - (6) Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	4,358,273		4,357,458
Commercial Real Estate Portfolio Segment [Member] | (7) Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	92,245		69,023
Commercial Real Estate Portfolio Segment [Member] | (8) Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	105,690		84,365
Commercial Real Estate Portfolio Segment [Member] | (9) Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	0		0
Finance Leases Portfolio Segment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	585,673		635,629		$ 618,343
Finance Leases Portfolio Segment [Member] | (1) - (6) Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	567,168		618,084
Finance Leases Portfolio Segment [Member] | (7) Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	3,802		1,324
Finance Leases Portfolio Segment [Member] | (8) Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	14,703		16,221
Finance Leases Portfolio Segment [Member] | (9) Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases	$ 0		$ 0

[1] At June 30, 2017, the Company had pledged $6.2 billion of eligible residential, consumer and commercial loans as collateral to support borrowing capacity at the FHLB Boston and the FRB of Boston.

[2] Loans and leases include net deferred fees and net premiums/discounts of $18.8 million and $17.3 million at June 30, 2017 and December 31, 2016, respectively.