Portfolio loans - Risk Grading (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Pass and watch
Loans
Portfolio loans receivable	$ 5,400,000	$ 5,300
Special mention, substandard and doubtful
Loans
Portfolio loans receivable	279,000	193,800
Pass
Loans
Portfolio loans receivable	5,049,360	5,022,249
Watch
Loans
Portfolio loans receivable	302,035	320,016
Special Mention
Loans
Limit above which loans are annually reviewed	350
Portfolio loans receivable	136,504	81,559
Substandard
Loans
Portfolio loans receivable	102,146	87,628
Doubtful
Loans
Portfolio loans receivable	40,395	24,624
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	1,205,074	1,175,421
Commercial | Watch
Loans
Limit above which loans are annually reviewed	1,000
Portfolio loans receivable	136,262	141,776
Commercial | Special Mention
Loans
Portfolio loans receivable	51,840	51,366
Commercial | Substandard
Loans
Portfolio loans receivable	48,266	43,933
Commercial | Doubtful
Loans
Portfolio loans receivable	20,994	5,285
Commercial real estate | Maximum
Loans
Limit where loans are processed through an expedited underwriting process	1,000
Commercial real estate | Pass
Loans
Portfolio loans receivable	2,098,441	2,169,420
Commercial real estate | Watch
Loans
Portfolio loans receivable	146,634	130,056
Commercial real estate | Special Mention
Loans
Portfolio loans receivable	76,941	21,151
Commercial real estate | Substandard
Loans
Portfolio loans receivable	46,267	36,482
Commercial real estate | Doubtful
Loans
Portfolio loans receivable	13,417	11,997
Real estate construction | Pass
Loans
Portfolio loans receivable	289,134	212,952
Real estate construction | Watch
Loans
Portfolio loans receivable	14,977	41,292
Real estate construction | Special Mention
Loans
Portfolio loans receivable	3,110	3,880
Real estate construction | Substandard
Loans
Portfolio loans receivable	1,923	3,071
Real estate construction | Doubtful
Loans
Portfolio loans receivable	21	608
Retail real estate | Pass
Loans
Portfolio loans receivable	1,427,792	1,436,156
Retail real estate | Watch
Loans
Portfolio loans receivable	4,162	6,883
Retail real estate | Special Mention
Loans
Portfolio loans receivable	4,613	5,162
Retail real estate | Substandard
Loans
Portfolio loans receivable	5,690	4,135
Retail real estate | Doubtful
Loans
Portfolio loans receivable	5,931	6,714
Retail other | Pass
Loans
Portfolio loans receivable	28,919	28,300
Retail other | Watch
Loans
Portfolio loans receivable		9
Retail other | Substandard
Loans
Portfolio loans receivable		7
Retail other | Doubtful
Loans
Portfolio loans receivable	$ 32	$ 20