Loans receivable (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Changes in allowance for losses on allocated loans receivable
Balance at the beginning of the period	$ 67,518	$ 71,348
Provision	900	7,485
Charge-offs	(4,454)	(4,249)
Recoveries	3,334	1,650
Balance at the end of the period	67,298	76,234
Personal Banking
Changes in allowance for losses on allocated loans receivable
Balance at the beginning of the period	16,249	20,607
Provision	775	1,256
Charge-offs	(2,617)	(2,547)
Recoveries	548	389
Balance at the end of the period	14,955	19,705
Personal Banking | Residential mortgage loans
Changes in allowance for losses on allocated loans receivable
Balance at the beginning of the period	5,581	7,875
Provision	(282)	35
Charge-offs	(335)	(459)
Recoveries	113	16
Balance at the end of the period	5,077	7,467
Personal Banking | Home equity loans
Changes in allowance for losses on allocated loans receivable
Balance at the beginning of the period	4,550	7,245
Provision	(213)	37
Charge-offs	(342)	(372)
Recoveries	48	48
Balance at the end of the period	4,043	6,958
Personal Banking | Other consumer loans
Changes in allowance for losses on allocated loans receivable
Balance at the beginning of the period	6,118	5,487
Provision	1,270	1,184
Charge-offs	(1,940)	(1,716)
Recoveries	387	325
Balance at the end of the period	5,835	5,280
Business Banking
Changes in allowance for losses on allocated loans receivable
Balance at the beginning of the period	46,904	46,079
Provision	512	6,740
Charge-offs	(1,837)	(1,702)
Recoveries	2,786	1,261
Balance at the end of the period	48,365	52,378
Business Banking | Commercial real estate loans
Changes in allowance for losses on allocated loans receivable
Balance at the beginning of the period	33,389	35,199
Provision	242	1,321
Charge-offs	(1,113)	(932)
Recoveries	734	621
Balance at the end of the period	33,252	36,209
Business Banking | Commercial loans
Changes in allowance for losses on allocated loans receivable
Balance at the beginning of the period	13,515	10,880
Provision	270	5,419
Charge-offs	(724)	(770)
Recoveries	2,052	640
Balance at the end of the period	15,113	16,169
Unallocated
Changes in allowance for losses on allocated loans receivable
Balance at the beginning of the period	4,365	4,662
Provision	(387)	(511)
Balance at the end of the period	$ 3,978	$ 4,151