Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits
Components of net periodic benefit cost
Total net periodic benefit cost	$ 2,430	$ 2,339
Pension Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	17	174
Pension Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	2,044	1,794
Expected return on plan assets	(1,195)	(1,240)
Recognized net actuarial loss	1,564	1,611
Other Benefits
Components of net periodic benefit cost
Total net periodic benefit cost	182	290
Other Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	159	212
Other Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	206	185
Prior service credit	(107)	$ (107)
Recognized net actuarial loss	$ (76)