Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits
Components of net periodic benefit cost
Total net periodic benefit cost	$ 2,339	$ 2,956
Pension Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	174	158
Pension Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	1,794	2,041
Expected return on plan assets	(1,240)	(1,242)
Recognized net actuarial loss	1,611	1,999
Other Benefits
Components of net periodic benefit cost
Total net periodic benefit cost	290	273
Other Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	212	179
Other Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	185	201
Prior service credit	$ (107)	$ (107)