Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Components of net periodic benefit cost
Total net periodic benefit cost	$ 2,339	$ 2,956	$ 4,678	$ 5,912
Pension Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	174	157	348	315
Pension Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	1,795	2,042	3,589	4,083
Expected return on plan assets	(1,239)	(1,242)	(2,479)	(2,484)
Recognized net actuarial loss	1,609	1,999	3,220	3,998
Other Benefits
Components of net periodic benefit cost
Total net periodic benefit cost	290	272	580	545
Other Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	213	179	425	358
Other Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	184	201	369	402
Prior service credit			$ (214)	$ (215)
Recognized net actuarial loss	$ (107)	$ (108)