Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits
Components of net periodic benefit cost
Total net periodic benefit cost	$ 2,430	$ 2,339	$ 4,860	$ 4,678
Pension Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	17	174	34	348
Pension Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	2,044	1,795	4,088	3,589
Expected return on plan assets	(1,195)	(1,239)	(2,390)	(2,479)
Recognized net actuarial loss	1,564	1,609	3,128	3,220
Other Benefits
Components of net periodic benefit cost
Total net periodic benefit cost	182	290	364	580
Other Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	159	213	318	425
Other Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	206	184	412	369
Prior service credit	(107)	$ (107)	(214)	$ (214)
Recognized net actuarial loss	$ (76)		$ (152)