Employee Benefits (Changes in Funded Status of Plan - Other Postretirement Benefit Plans) (Details) - Other Postretirement Benefit Plan $ in Thousands		12 Months Ended
		Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 01, 2017
Change in projected benefit obligation:
Benefit obligation at end of prior year		$ 73,515	$ 92,643
Service cost		2,236	2,769	$ 3,454
Interest cost		2,723	3,167	4,035
Actuarial (gain) loss		(8,319)	10,751
Amendment	[1]	0	(32,697)
Benefits paid		(4,087)	(4,428)
Retiree contributions		1,222	1,310
Benefit obligation at end of year		67,290	73,515	92,643
Change in plan assets:
Fair value of plan assets at end of prior year		39,115	38,090
Actual return on plan assets		4,173	2,443
Employer contribution		450	1,700
Benefits paid		(4,087)	(4,428)
Retiree contributions		1,222	1,310
Fair value of plan assets at end of year		40,873	39,115	$ 38,090
Funded status at end of year		$ (26,417)	$ (34,400)
$1,000 or $2,250 deductible plan options | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Age Eligible for Benefit					65
Medicare advantage plan | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Age Eligible for Benefit					65

[1] During October 2016, the Company approved and communicated a plan amendment that resulted in a remeasurement of the Company's Other Post-retirement Benefit Plan. Effective January 1, 2017, retirees and dependents that are less than 65 years of age are offered a choice between a $1,000 and $2,250 deductible plan. Additionally, retirees and dependents that are 65 years of age or greater were covered by a fully insured Medicare advantage plan.