Benefit Plans - Net periodic benefit cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Components of net periodic benefit cost
Service cost		$ 14
Interest cost	$ 6,519	8,261
Total net periodic benefit cost	7,525	10,685	$ 10,100
Pension Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost		14	696
Pension Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	6,519	8,261	7,362
Expected return on plan assets	(4,800)	(4,585)	(5,273)
Recognized net actuarial loss (gain)	5,806	6,995	7,315
Pension Benefits | Funded Plan
Components of net periodic benefit cost
Interest cost	2,946	3,808	3,420
Expected return on plan assets	(4,800)	(4,585)	(5,273)
Recognized net actuarial loss (gain)	1,421	3,714	2,600
Total net periodic benefit cost	(433)	2,937	747
Other Benefits
Components of net periodic benefit cost
Service cost	768	710
Interest cost	640	808
Total net periodic benefit cost	1,146	704	1,060
Other Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	768	710	750
Other Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	640	808	739
Prior service credit	(51)	(429)	$ (429)
Recognized net actuarial loss (gain)	$ (211)	$ (385)