Pension Benefit and Retirement Health and Life Insurance Benefits (Fair Value of Net Assets by Asset Category) (Details) - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Pension assets	$ 180,056	$ 171,778	$ 171,007
Pooled separate accounts
Defined Benefit Plan Disclosure [Line Items]
Pension assets	4,610	7,587
Fixed income bonds
Defined Benefit Plan Disclosure [Line Items]
Pension assets	162,934	111,070
Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Pension assets	6,223	44,054
Guaranteed deposit account
Defined Benefit Plan Disclosure [Line Items]
Pension assets	$ 6,289	$ 9,067