Bank segment - Credit risk profile - assigned grades (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	$ 4,742,680	$ 4,671,577
Commercial real estate
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	766,522	733,106
Commercial real estate | Pass
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	667,555	630,877
Commercial real estate | Special mention
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	46,283	49,347
Commercial real estate | Substandard
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	52,684	52,882
Commercial real estate | Doubtful
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	0	0
Commercial real estate | Loss
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	0	0
Commercial construction
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	114,535	108,273
Commercial construction | Pass
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	89,802	83,757
Commercial construction | Special mention
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	22,500	22,500
Commercial construction | Substandard
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	2,233	2,016
Commercial construction | Doubtful
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	0	0
Commercial construction | Loss
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	0	0
Commercial loans
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	568,371	544,828
Commercial loans | Pass
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	518,819	492,942
Commercial loans | Special mention
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	27,876	27,997
Commercial loans | Substandard
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	21,676	23,421
Commercial loans | Doubtful
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	0	468
Commercial loans | Loss
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	$ 0	$ 0