Employee Benefit Plans (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) pension_plan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, expenses	$ 9,900	$ 9,600	$ 8,900
Number of Union Pension Plans | pension_plan	2
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year	5.40%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate	4.50%
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	67.00%	67.00%
Fixed Income Securities [Member] | Scenario, Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	70.00%
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	30.00%	30.00%
Equity Securities [Member] | Scenario, Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	30.00%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation	$ 297,856	$ 225,970	209,591
Defined Benefit Plan, Fair Value of Plan Assets	198,686	166,906	142,742
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	(59,686)	(1,050)
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	(7,000)
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year	(200)
Contributions estimated to be paid	28,100
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation	12,760	13,011	11,314
Defined Benefit Plan, Fair Value of Plan Assets	0	0	$ 0
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	535	$ (1,481)
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	(600)
Contributions estimated to be paid	500
Salaried Plan and IUOE [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation	154,400
Defined Benefit Plan, Fair Value of Plan Assets	$ 145,900