Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits
Components of net periodic benefit cost
Total net periodic benefit cost	$ 2,525	$ 2,956	$ 7,203	$ 8,868
Pension Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	174	157	522	472
Pension Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	1,840	2,041	5,429	6,124
Expected return on plan assets	(1,318)	(1,242)	(3,797)	(3,726)
Recognized net actuarial loss	1,829	2,000	5,049	5,998
Other Benefits
Components of net periodic benefit cost
Total net periodic benefit cost	265	273	845	818
Other Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	80	179	505	537
Other Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	293	201	662	603
Prior service credit	$ (108)	$ (107)	$ (322)	$ (322)