Portfolio loans - Risk Grading (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Pass and watch
Loans
Portfolio loans receivable	$ 3,790,000	$ 3,720,000
Special mention, substandard and doubtful
Loans
Portfolio loans receivable	135,800	165,500
Pass
Loans
Portfolio loans receivable	3,506,163	3,532,612
Watch
Loans
Portfolio loans receivable	284,238	185,954
Special Mention
Loans
Limit above which loans are annually reviewed	350
Portfolio loans receivable	50,330	78,554
Substandard
Loans
Portfolio loans receivable	66,563	65,478
Doubtful
Loans
Portfolio loans receivable	18,935	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	768,845	826,163
Commercial | Watch
Loans
Limit above which loans are annually reviewed	1,000
Portfolio loans receivable	125,169	70,260
Commercial | Special Mention
Loans
Portfolio loans receivable	25,684	26,951
Commercial | Substandard
Loans
Portfolio loans receivable	30,994	26,941
Commercial | Doubtful
Loans
Portfolio loans receivable	9,456	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	1,526,365	1,507,513
Commercial real estate | Watch
Loans
Portfolio loans receivable	112,921	69,145
Commercial real estate | Special Mention
Loans
Portfolio loans receivable	15,813	40,775
Commercial real estate | Substandard
Loans
Portfolio loans receivable	29,958	35,385
Commercial real estate | Doubtful
Loans
Portfolio loans receivable	5,540	5,154
Real estate construction | Pass
Loans
Portfolio loans receivable	133,371	134,574
Real estate construction | Watch
Loans
Portfolio loans receivable	40,591	39,936
Real estate construction | Special Mention
Loans
Portfolio loans receivable	5,319	8,033
Real estate construction | Substandard
Loans
Portfolio loans receivable	3,542	994
Real estate construction | Doubtful
Loans
Portfolio loans receivable	32	47
Retail real estate | Pass
Loans
Portfolio loans receivable	1,065,428	1,050,671
Retail real estate | Watch
Loans
Portfolio loans receivable	5,557	6,586
Retail real estate | Special Mention
Loans
Portfolio loans receivable	3,514	2,793
Retail real estate | Substandard
Loans
Portfolio loans receivable	2,069	2,158
Retail real estate | Doubtful
Loans
Portfolio loans receivable	3,860	4,484
Retail other | Pass
Loans
Portfolio loans receivable	12,154	13,691
Retail other | Watch
Loans
Portfolio loans receivable		27
Retail other | Special Mention
Loans
Portfolio loans receivable		2
Retail other | Doubtful
Loans
Portfolio loans receivable	$ 47	$ 53