Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 696	$ 629
Interest cost	7,362	8,297
Total net periodic benefit cost	10,100	12,548	$ 12,659
Pension Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	696	629	944
Pension Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	7,362	8,297	8,784
Expected return on plan assets	(5,273)	(5,003)	(4,698)
Recognized net actuarial loss	7,315	8,625	7,629
Pension Benefits | Funded Plan
Components of net periodic benefit cost
Interest cost	3,420	3,922	4,182
Expected return on plan assets	(5,273)	(5,003)	(4,698)
Recognized net actuarial loss	2,600	4,316	3,443
Total net periodic benefit cost	747	3,235	2,927
Other Benefits
Components of net periodic benefit cost
Service cost	750	738
Interest cost	739	776
Total net periodic benefit cost	1,060	1,085	1,080
Other Benefits | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	750	738	697
Other Benefits | Other noninterest expense
Components of net periodic benefit cost
Interest cost	739	776	812
Prior service credit	$ (429)	$ (429)	$ (429)