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, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Pension assets	$ 171,778	$ 171,007	$ 170,600
Pooled separate accounts
Defined Benefit Plan Disclosure [Line Items]
Pension assets	7,587	6,782
Fixed income bonds
Defined Benefit Plan Disclosure [Line Items]
Pension assets	111,070	110,427
Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Pension assets	44,054	43,454
Guaranteed deposit account
Defined Benefit Plan Disclosure [Line Items]
Pension assets	$ 9,067	$ 10,344