Benefit plans (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Benefit Plans [Line Items]
Defined Contribution Plan, Vesting Period	5 years
Contributions to 401(k) profit sharing plan	$ 700,000	$ 500,000	$ 500,000
Bank owned life insurance	53,449,000	36,567,000
Deferred Compensation, Excluding Share-based Payments and Retirement Benefits
Benefit Plans [Line Items]
Other post-retirement defined benefit plan, liabilities	7,400,000	3,900,000
Deferred compensation arrangement, individual, compensation expense	300,000	100,000	200,000
Supplemental Employee Retirement Plan
Benefit Plans [Line Items]
Other post-retirement defined benefit plan, liabilities	18,700,000	12,700,000
Deferred compensation arrangement, individual, compensation expense	1,000,000	800,000	(500,000)
General Account BOLI
Benefit Plans [Line Items]
Bank owned life insurance	25,100,000	8,700,000
Separate Account BOLI
Benefit Plans [Line Items]
Bank owned life insurance	$ 28,300,000	$ 27,900,000