Portfolio loans - Risk Grading (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Pass and watch
Loans
Portfolio loans receivable	$ 4,900,000	$ 3,700,000
Special mention, substandard and doubtful
Loans
Portfolio loans receivable	174,100	165,500
Pass
Loans
Portfolio loans receivable	4,636,358	3,532,612
Watch
Loans
Portfolio loans receivable	290,444	185,954
Special Mention
Loans
Limit above which loans are annually reviewed	350
Portfolio loans receivable	73,257	78,554
Substandard
Loans
Portfolio loans receivable	73,379	65,478
Doubtful
Loans
Portfolio loans receivable	27,430	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	1,107,709	826,163
Commercial | Watch
Loans
Limit above which loans are annually reviewed	1,000
Portfolio loans receivable	134,287	70,260
Commercial | Special Mention
Loans
Portfolio loans receivable	38,283	26,951
Commercial | Substandard
Loans
Portfolio loans receivable	35,885	26,941
Commercial | Doubtful
Loans
Portfolio loans receivable	7,215	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	2,050,678	1,507,513
Commercial real estate | Watch
Loans
Portfolio loans receivable	108,687	69,145
Commercial real estate | Special Mention
Loans
Portfolio loans receivable	23,040	40,775
Commercial real estate | Substandard
Loans
Portfolio loans receivable	31,374	35,385
Commercial real estate | Doubtful
Loans
Portfolio loans receivable	13,313	5,154
Real estate construction | Pass
Loans
Portfolio loans receivable	198,095	134,574
Real estate construction | Watch
Loans
Portfolio loans receivable	41,056	39,936
Real estate construction | Special Mention
Loans
Portfolio loans receivable	5,750	8,033
Real estate construction | Substandard
Loans
Portfolio loans receivable	3,119	994
Real estate construction | Doubtful
Loans
Portfolio loans receivable	610	47
Retail real estate | Pass
Loans
Portfolio loans receivable	1,264,840	1,050,671
Retail real estate | Watch
Loans
Portfolio loans receivable	6,414	6,586
Retail real estate | Special Mention
Loans
Portfolio loans receivable	6,184	2,793
Retail real estate | Substandard
Loans
Portfolio loans receivable	3,001	2,158
Retail real estate | Doubtful
Loans
Portfolio loans receivable	6,247	4,484
Retail other | Pass
Loans
Portfolio loans receivable	15,036	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	$ 45	$ 53