Pension Benefit and Retirement Health and Life Insurance Benefits (Assets Carried at Fair Value by Level) (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
Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension assets	6,223	44,054
Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension assets	167,544	118,657
Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension assets	6,289	9,067
Pooled separate accounts
Defined Benefit Plan Disclosure [Line Items]
Pension assets	4,610	7,587
Pooled separate accounts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0
Pooled separate accounts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension assets	4,610	7,587
Pooled separate accounts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0
Fixed income bonds
Defined Benefit Plan Disclosure [Line Items]
Pension assets	162,934	111,070
Fixed income bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0
Fixed income bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension assets	162,934	111,070
Fixed income bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0
Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Pension assets	6,223	44,054
Mutual funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension assets	6,223	44,054
Mutual funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0
Mutual funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0
Guaranteed deposit account
Defined Benefit Plan Disclosure [Line Items]
Pension assets	6,289	9,067
Guaranteed deposit account | Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0
Guaranteed deposit account | Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0
Guaranteed deposit account | Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension assets	$ 6,289	$ 9,067