Accounting changes and new accounting guidance - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Jan. 01, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Minimum loan amount required to measure impairment allowance	$ 1,000,000
ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment for the cumulative effect of applying the ASU		$ 40,000,000
Retained earnings | ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment for the cumulative effect of applying the ASU		$ 45,000,000
Commercial Loan
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Threshold period past due for nonaccrual status	90 days
First Lien Residential Mortgage
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Threshold period past due subject to impairment test	90 days
Second Lien Residential Mortgage
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Threshold period past due subject to further impairment testing	90 days