Bank segment - Credit risk profile - assigned grades (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	$ 5,180,378	$ 5,120,664
Commercial Portfolio Segment
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		1,566,109
Commercial Portfolio Segment | Pass
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		1,446,720
Commercial Portfolio Segment | Special Mention
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		34,372
Commercial Portfolio Segment | Substandard
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		85,017
Commercial Portfolio Segment | Doubtful
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		0
Commercial Portfolio Segment | Loss
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		0
Commercial real estate
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	852,120	824,830
Commercial real estate | Pass
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		756,747
Commercial real estate | Special Mention
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		4,451
Commercial real estate | Substandard
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		63,632
Commercial real estate | Doubtful
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		0
Commercial real estate | Loss
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		0
Commercial construction
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	79,377	70,605
Commercial construction | Pass
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		68,316
Commercial construction | Special Mention
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		0
Commercial construction | Substandard
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		2,289
Commercial construction | Doubtful
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		0
Commercial construction | Loss
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		0
Commercial loans
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross	$ 722,647	670,674
Commercial loans | Pass
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		621,657
Commercial loans | Special Mention
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		29,921
Commercial loans | Substandard
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		19,096
Commercial loans | Doubtful
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		0
Commercial loans | Loss
Credit risk profile by internally assigned grade for loans
Loans and leases receivable before fees, gross		$ 0