Fair Value Measurements (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Loan 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	$ 345,576	$ 280,486
Fair Value
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	348,221	284,188
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	308,645	241,111
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	310,338	244,101
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	36,931	39,375
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	$ 37,883	$ 40,087