Benefit plans (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Benefit Plans [Line Items]
Defined Contribution Plan, Vesting Period	5 years
Contributions to 401(k) profit sharing plan	$ 900	$ 700	$ 500
Bank owned life insurance	54,450	53,449
Deferred Compensation, Excluding Share-based Payments and Retirement Benefits
Benefit Plans [Line Items]
Other post-retirement defined benefit plan, liabilities	7,400	7,400
Deferred compensation arrangement, individual, compensation expense	400	300	100
Supplemental Employee Retirement Plan
Benefit Plans [Line Items]
Other post-retirement defined benefit plan, liabilities	19,100	18,700
Deferred compensation arrangement, individual, compensation expense	1,400	1,000	$ 800
General Account BOLI
Benefit Plans [Line Items]
Bank owned life insurance	25,900	25,100
Separate Account BOLI
Benefit Plans [Line Items]
Bank owned life insurance	$ 28,600	$ 28,300