Pension Benefit and Retirement Health and Life Insurance Benefits (Assets Carried at Fair Value by Level) (Details) - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Pension assets	$ 171,778	$ 171,007	$ 170,600
Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension assets	44,054	43,454
Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension assets	118,657	117,209
Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension assets	9,067	10,344
Pooled separate accounts
Defined Benefit Plan Disclosure [Line Items]
Pension assets	7,587	6,782
Pooled separate accounts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0	0
Pooled separate accounts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension assets	7,587	6,782
Pooled separate accounts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0	0
Fixed income bonds
Defined Benefit Plan Disclosure [Line Items]
Pension assets	111,070	110,427
Fixed income bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0	0
Fixed income bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension assets	111,070	110,427
Fixed income bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0	0
Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Pension assets	44,054	43,454
Mutual funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension assets	44,054	43,454
Mutual funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0	0
Mutual funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0	0
Guaranteed deposit account
Defined Benefit Plan Disclosure [Line Items]
Pension assets	9,067	10,344
Guaranteed deposit account | Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0	0
Guaranteed deposit account | Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension assets	0	0
Guaranteed deposit account | Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension assets	$ 9,067	$ 10,344