Fair Value Measurements (Fair Value of Long-term Borrowings Outstanding) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt instrument term	15 years
Carrying Amount
Debt Instrument [Line Items]
Fair value of long-term borrowings	$ 288,942	$ 306,421
Carrying Amount | Fixed rate long-term notes payable
Debt Instrument [Line Items]
Fair value of long-term borrowings	261,618	275,989
Carrying Amount | Debenture bonds
Debt Instrument [Line Items]
Fair value of long-term borrowings	27,324	30,432
Fair Value | Level 2
Debt Instrument [Line Items]
Fair value of long-term borrowings	282,601	304,792
Fair Value | Level 2 | Fixed rate long-term notes payable
Debt Instrument [Line Items]
Fair value of long-term borrowings	256,447	275,340
Fair Value | Level 2 | Debenture bonds
Debt Instrument [Line Items]
Fair value of long-term borrowings	$ 26,154	$ 29,452