Pension Benefits and Other Postretirement Benefit Plans (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1,841	$ 1,893	$ 3,682	$ 3,786
Expected return on plan assets	(2,309)	(2,706)	(4,618)	(5,412)
Amortization of net loss (gain)	433	447	866	894
Net periodic (benefit) cost	(35)	(366)	(70)	(732)
Retirement Health and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	17	37	56	74
Interest cost	13	19	31	38
Amortization of prior service cost (credit)	(407)	(373)	(780)	(746)
Amortization of net loss (gain)	(17)	(19)	(31)	(38)
Net periodic (benefit) cost	$ (394)	$ (336)	(724)	(672)
Bear Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, contributions by employer					$ 300
Defined benefit pension plan, required employer contribution			100	$ 100
Defined benefit plan, estimated future employer contribution			$ 200