Pension Benefit and Retirement Health and Life Insurance Benefits (Obligations and Funded Status) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 177,696	$ 182,359
Service cost	0	0
Interest cost	7,356	7,530	$ 7,523
Actuarial (gain) loss	9,601	(3,621)
Benefit payments	(8,893)	(8,572)
Plan Amendment	0	0
Benefit obligation at end of year	185,760	177,696	182,359
Change in plan assets:
Fair value of plan assets at the beginning of the year	171,778	171,007
Actual return on plan assets	16,799	8,999
Employer contributions	372	344
Benefit payments	(8,893)	(8,572)
Fair value of plan assets at the end of the year	180,056	171,778	171,007
Unfunded status	(5,704)	(5,918)
Retirement Health and Life Insurance Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	2,504	2,722
Service cost	80	133	411
Interest cost	71	75	216
Actuarial (gain) loss	460	72
Benefit payments	(533)	(498)
Plan Amendment	(545)	0
Benefit obligation at end of year	2,037	2,504	2,722
Change in plan assets:
Fair value of plan assets at the beginning of the year	0	0
Actual return on plan assets	0	0
Employer contributions	533	498
Benefit payments	(533)	(498)
Fair value of plan assets at the end of the year	0	0	$ 0
Unfunded status	$ (2,037)	$ (2,504)