Portfolio loans - Risk Grades (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Pass
Loans
Portfolio loans receivable	$ 5,866,151	$ 4,976,708
Watch
Loans
Portfolio loans receivable	417,626	330,650
Special Mention
Loans
Portfolio loans receivable	140,785	142,839
Substandard
Loans
Portfolio loans receivable	86,033	86,060
Substandard Non-accrual
Loans
Portfolio loans receivable	32,816	34,997
Commercial | Pass
Loans
Portfolio loans receivable	1,378,041	1,126,257
Commercial | Watch
Loans
Portfolio loans receivable	188,282	172,449
Commercial | Special Mention
Loans
Portfolio loans receivable	48,181	47,000
Commercial | Substandard
Loans
Portfolio loans receivable	45,599	42,532
Commercial | Substandard Non-accrual
Loans
Portfolio loans receivable	10,647	17,953
Commercial real estate | Pass
Loans
Portfolio loans receivable	2,367,741	2,106,711
Commercial real estate | Watch
Loans
Portfolio loans receivable	178,859	137,214
Commercial real estate | Special Mention
Loans
Portfolio loans receivable	85,837	85,148
Commercial real estate | Substandard
Loans
Portfolio loans receivable	29,926	36,205
Commercial real estate | Substandard Non-accrual
Loans
Portfolio loans receivable	12,780	10,298
Real estate construction | Pass
Loans
Portfolio loans receivable	394,416	268,069
Real estate construction | Watch
Loans
Portfolio loans receivable	34,348	14,562
Real estate construction | Special Mention
Loans
Portfolio loans receivable	450	3,899
Real estate construction | Substandard
Loans
Portfolio loans receivable	1,273	1,888
Real estate construction | Substandard Non-accrual
Loans
Portfolio loans receivable	826	18
Retail real estate | Pass
Loans
Portfolio loans receivable	1,674,105	1,448,964
Retail real estate | Watch
Loans
Portfolio loans receivable	16,038	6,425
Retail real estate | Special Mention
Loans
Portfolio loans receivable	6,317	6,792
Retail real estate | Substandard
Loans
Portfolio loans receivable	9,235	5,435
Retail real estate | Substandard Non-accrual
Loans
Portfolio loans receivable	8,529	6,698
Retail other | Pass
Loans
Portfolio loans receivable	51,848	26,707
Retail other | Watch
Loans
Portfolio loans receivable	99
Retail other | Substandard Non-accrual
Loans
Portfolio loans receivable	$ 34	$ 30