Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 14	$ 696
Interest cost	8,261	7,362
Total net periodic benefit cost	10,685	10,100	$ 12,548
Pension Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	14	696	629
Pension Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	8,261	7,362	8,297
Expected return on plan assets	(4,585)	(5,273)	(5,003)
Recognized net actuarial loss (gain)	6,995	7,315	8,625
Pension Benefits | Funded Plan
Components of net periodic benefit cost
Interest cost	3,808	3,420	3,922
Expected return on plan assets	(4,585)	(5,273)	(5,003)
Recognized net actuarial loss (gain)	3,714	2,600	4,316
Total net periodic benefit cost	2,937	747	3,235
Other Benefits
Components of net periodic benefit cost
Service cost	710	750
Interest cost	808	739
Total net periodic benefit cost	704	1,060	1,085
Other Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	710	750	738
Other Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	808	739	776
Prior service credit	(429)	$ (429)	$ (429)
Recognized net actuarial loss (gain)	$ (385)