Fair Value Measurements (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Debt instrument term	15 years
Carrying Amount
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	$ 306,421	$ 345,576
Fair Value
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	304,792	348,221
Level 2 | Fixed rate long-term notes payable | Carrying Amount
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	275,989	308,645
Level 2 | Fixed rate long-term notes payable | Fair Value
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	275,340	310,338
Level 2 | Debenture bonds | Carrying Amount
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	30,432	36,931
Level 2 | Debenture bonds | Fair Value
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	$ 29,452	$ 37,883