Basis of presentation and summary of significant accounting policies (Agreement with First Interstate BancSystem, Inc) (Details) - First Interstate BancSystem, Inc - First Interstate Merger - USD ($)		12 Months Ended
	Nov. 17, 2016	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Total consideration	$ 590,500,000
Share price of acquisition	$ 7.62
Share price	$ 38.40
Number of securities called by each right (in shares)	0.14864
Exercise price of right (in dollars per share)	$ 1.91
Percentage of capital stock acquired	20.00%
Assets threshold of debit interchange limitations	$ 10,000,000,000
Forecast
Business Acquisition [Line Items]
Long-term annual earnings per share accretion		10.00%	8.00%