Portfolio loans - Risk Grading (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Pass and watch
Loans
Portfolio loans receivable	$ 3,720,000	$ 3,720,000
Special mention, substandard and doubtful
Loans
Portfolio loans receivable	163,000	165,500
Pass
Loans
Portfolio loans receivable	3,524,850	3,532,612
Watch
Loans
Portfolio loans receivable	192,481	185,954
Special Mention
Loans
Limit above which loans are annually reviewed	350
Portfolio loans receivable	71,210	78,554
Substandard
Loans
Portfolio loans receivable	71,211	65,478
Doubtful
Loans
Portfolio loans receivable	20,544	21,423
Commercial | Pass and watch
Loans
Limit where loans are processed through an expedited underwriting process	350
Commercial | Pass and watch | Minimum
Loans
Limit where loans are processed through an expedited underwriting process	350
Commercial | Pass and watch | Maximum
Loans
Limit where loans are processed through an expedited underwriting process	1,000
Commercial | Pass
Loans
Portfolio loans receivable	836,897	826,163
Commercial | Watch
Loans
Limit above which loans are annually reviewed	1,000
Portfolio loans receivable	69,245	70,260
Commercial | Special Mention
Loans
Portfolio loans receivable	25,455	26,951
Commercial | Substandard
Loans
Portfolio loans receivable	29,371	26,941
Commercial | Doubtful
Loans
Portfolio loans receivable	11,275	11,685
Commercial real estate
Loans
Limit where loans are processed through an expedited underwriting process	1,000
Commercial real estate | Pass
Loans
Portfolio loans receivable	1,505,049	1,507,513
Commercial real estate | Watch
Loans
Portfolio loans receivable	80,352	69,145
Commercial real estate | Special Mention
Loans
Portfolio loans receivable	35,734	40,775
Commercial real estate | Substandard
Loans
Portfolio loans receivable	36,364	35,385
Commercial real estate | Doubtful
Loans
Portfolio loans receivable	4,912	5,154
Real estate construction | Pass
Loans
Portfolio loans receivable	123,920	134,574
Real estate construction | Watch
Loans
Portfolio loans receivable	36,583	39,936
Real estate construction | Special Mention
Loans
Portfolio loans receivable	7,156	8,033
Real estate construction | Substandard
Loans
Portfolio loans receivable	3,146	994
Real estate construction | Doubtful
Loans
Portfolio loans receivable	34	47
Retail real estate | Pass
Loans
Portfolio loans receivable	1,046,197	1,050,671
Retail real estate | Watch
Loans
Portfolio loans receivable	6,287	6,586
Retail real estate | Special Mention
Loans
Portfolio loans receivable	2,865	2,793
Retail real estate | Substandard
Loans
Portfolio loans receivable	2,330	2,158
Retail real estate | Doubtful
Loans
Portfolio loans receivable	4,275	4,484
Retail other | Pass
Loans
Portfolio loans receivable	12,787	13,691
Retail other | Watch
Loans
Portfolio loans receivable	14	27
Retail other | Special Mention
Loans
Portfolio loans receivable		2
Retail other | Doubtful
Loans
Portfolio loans receivable	$ 48	$ 53