Pension Benefit and Retirement Health and Life Insurance Benefits (Summary of Changes in Fair Value of Guaranteed Deposit Account's Level 3 Assets) (Details) - Pension Benefits - Guaranteed deposit account - Level 3 $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of year	$ 9,067
Unrealized gains relating to instruments still held at the reporting date	216
Purchases, sales, issuances and settlements (net)	(2,994)
Balance at end of year	$ 6,289