Portfolio loans - Risk Grades (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Pass
Loans
Portfolio loans receivable	$ 5,850,714	$ 4,976,708
Watch
Loans
Portfolio loans receivable	383,313	330,650
Special Mention
Loans
Portfolio loans receivable	158,764	142,839
Substandard
Loans
Portfolio loans receivable	84,150	86,060
Substandard Non-accrual
Loans
Portfolio loans receivable	36,230	34,997
Commercial | Pass
Loans
Portfolio loans receivable	1,359,640	1,126,257
Commercial | Watch
Loans
Portfolio loans receivable	180,954	172,449
Commercial | Special Mention
Loans
Portfolio loans receivable	59,595	47,000
Commercial | Substandard
Loans
Portfolio loans receivable	41,525	42,532
Commercial | Substandard Non-accrual
Loans
Portfolio loans receivable	16,243	17,953
Commercial real estate | Pass
Loans
Portfolio loans receivable	2,406,213	2,106,711
Commercial real estate | Watch
Loans
Portfolio loans receivable	168,415	137,214
Commercial real estate | Special Mention
Loans
Portfolio loans receivable	89,262	85,148
Commercial real estate | Substandard
Loans
Portfolio loans receivable	33,585	36,205
Commercial real estate | Substandard Non-accrual
Loans
Portfolio loans receivable	9,761	10,298
Real estate construction | Pass
Loans
Portfolio loans receivable	362,541	268,069
Real estate construction | Watch
Loans
Portfolio loans receivable	19,652	14,562
Real estate construction | Special Mention
Loans
Portfolio loans receivable	3,105	3,899
Real estate construction | Substandard
Loans
Portfolio loans receivable	1,346	1,888
Real estate construction | Substandard Non-accrual
Loans
Portfolio loans receivable	1,126	18
Retail real estate | Pass
Loans
Portfolio loans receivable	1,671,219	1,448,964
Retail real estate | Watch
Loans
Portfolio loans receivable	14,202	6,425
Retail real estate | Special Mention
Loans
Portfolio loans receivable	6,802	6,792
Retail real estate | Substandard
Loans
Portfolio loans receivable	7,678	5,435
Retail real estate | Substandard Non-accrual
Loans
Portfolio loans receivable	9,080	6,698
Retail other | Pass
Loans
Portfolio loans receivable	51,101	26,707
Retail other | Watch
Loans
Portfolio loans receivable	90
Retail other | Substandard
Loans
Portfolio loans receivable	16
Retail other | Substandard Non-accrual
Loans
Portfolio loans receivable	$ 20	$ 30