Pension and Other Postretirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Defined Benefit Pension Plans
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	$ 233	$ 277
Interest cost	1,302	1,358
Expected return on plan assets	(2,217)	(2,141)
Amortization of unrecognized net loss	578	572
Net periodic benefit cost (income)	(104)	66
Postretirement Benefit Plan
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	1	2
Interest cost	24	33
Amortization of prior service cost	0	29
Amortization of unrecognized net loss	(41)	(24)
Net periodic benefit cost (income)	$ (16)	$ 40