Employee Benefit Plans (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Prepayment amount	$ 7,200,000	$ 7,900,000
Deferred compensation	$ 1,229,294	907,836
Number of days of severance pay per year of service	14 days
Maximum number of days of severance pay	182 days
Number of years of service	13 years
NRECA Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Prepayment multiplier	2.50%
Prepayment, effect on billing rate	25.00%
Period of billing rate reduction related to prepayment	15 years
Prepayment amount		7,900,000
Prepayment amortization period	11 years
Multiemployer Plans, Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions to benefit plan	$ 5,900,000	6,700,000	$ 6,700,000
Union Health and Welfare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions to benefit plan	4,800,000	4,500,000	4,500,000
Non-Union Health And Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions to benefit plan	2,800,000	2,800,000	2,600,000
Money Purchase Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions to benefit plan	$ 141,800	$ 132,300	$ 133,600