Benefit plans (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Benefit Plans [Line Items]
Defined Contribution Plan, Vesting Period	5 years
Contributions to 401(k) profit sharing plan	$ 1,100	$ 900	$ 700
Bank owned life insurance	56,957	54,450
Supplemental Employee Retirement Plan
Benefit Plans [Line Items]
Other post-retirement defined benefit plan, liabilities	19,000	19,100
Deferred compensation arrangement, individual, compensation expense	900	1,400	1,000
Deferred Compensation, Excluding Share-based Payments and Retirement Benefits
Benefit Plans [Line Items]
Other post-retirement defined benefit plan, liabilities	7,200	7,400
Deferred compensation arrangement, individual, compensation expense	400	400	$ 300
General Account BOLI
Benefit Plans [Line Items]
Bank owned life insurance	28,000	25,900
Separate Account BOLI
Benefit Plans [Line Items]
Bank owned life insurance	$ 28,900	$ 28,600