Pension Benefits and Other Postretirement Benefit Plans (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1,841	$ 1,893
Expected return on plan assets	(2,309)	(2,706)
Amortization of net loss (gain)	433	447
Net periodic (benefit) cost	(35)	(366)
Retirement Health and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	39	37
Interest cost	18	19
Amortization of prior service cost (credit)	(373)	(373)
Amortization of net loss (gain)	(14)	(19)
Net periodic (benefit) cost	(330)	(336)
Bear Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, required employer contribution	100	$ 100
Defined benefit plan, contributions by employer			$ 300
Defined benefit plan, estimated future employer contribution	$ 300