Portfolio loans - Risk Grades (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Pass
Loans
Portfolio loans receivable	$ 5,986,984	$ 4,976,708
Watch
Loans
Portfolio loans receivable	395,242	330,650
Special Mention
Loans
Portfolio loans receivable	167,921	142,839
Substandard
Loans
Portfolio loans receivable	94,889	86,060
Substandard Non-accrual
Loans
Portfolio loans receivable	31,827	34,997
Commercial | Pass
Loans
Portfolio loans receivable	1,393,420	1,126,257
Commercial | Watch
Loans
Portfolio loans receivable	167,439	172,449
Commercial | Special Mention
Loans
Portfolio loans receivable	63,516	47,000
Commercial | Substandard
Loans
Portfolio loans receivable	47,765	42,532
Commercial | Substandard Non-accrual
Loans
Portfolio loans receivable	10,708	17,953
Commercial real estate | Pass
Loans
Portfolio loans receivable	2,474,248	2,106,711
Commercial real estate | Watch
Loans
Portfolio loans receivable	189,443	137,214
Commercial real estate | Special Mention
Loans
Portfolio loans receivable	91,689	85,148
Commercial real estate | Substandard
Loans
Portfolio loans receivable	38,018	36,205
Commercial real estate | Substandard Non-accrual
Loans
Portfolio loans receivable	11,852	10,298
Real estate construction | Pass
Loans
Portfolio loans receivable	397,377	268,069
Real estate construction | Watch
Loans
Portfolio loans receivable	24,050	14,562
Real estate construction | Special Mention
Loans
Portfolio loans receivable	5,151	3,899
Real estate construction | Substandard
Loans
Portfolio loans receivable	1,130	1,888
Real estate construction | Substandard Non-accrual
Loans
Portfolio loans receivable	611	18
Retail real estate | Pass
Loans
Portfolio loans receivable	1,670,187	1,448,964
Retail real estate | Watch
Loans
Portfolio loans receivable	14,231	6,425
Retail real estate | Special Mention
Loans
Portfolio loans receivable	7,565	6,792
Retail real estate | Substandard
Loans
Portfolio loans receivable	7,963	5,435
Retail real estate | Substandard Non-accrual
Loans
Portfolio loans receivable	8,591	6,698
Retail other | Pass
Loans
Portfolio loans receivable	51,752	26,707
Retail other | Watch
Loans
Portfolio loans receivable	79
Retail other | Substandard
Loans
Portfolio loans receivable	13
Retail other | Substandard Non-accrual
Loans
Portfolio loans receivable	$ 65	$ 30