Long-Term Debt and Short-Term Borrowings, Short-Term Borrowings (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Bank	Dec. 31, 2016 USD ($)
Line of Credit Facility [Abstract]
Borrowing capacity	$ 41,500
Number of banks in which unsecured line of credit maintained | Bank	4
Average borrowings outstanding	$ 3,132
Weighted average cost of borrowings	1.76%
Weighted average interest rate at year end	2.65%
Unsecured Line of Credit, First Note [Member]
Line of Credit Facility [Abstract]
Borrowing capacity	$ 13,000
Maturity period	2 years
Maturity date	May 31, 2019
Outstanding borrowings under line of credit	$ 5,389	$ 0
Unsecured Line of Credit, First Note [Member] | LIBOR [Member]
Line of Credit Facility [Abstract]
Basis adjustment	1.20%
Unsecured Line of Credit, Second Note [Member]
Line of Credit Facility [Abstract]
Borrowing capacity	$ 11,000
Maturity date	May 31, 2019
Outstanding borrowings under line of credit	$ 0	0
Unsecured Line of Credit, Second Note [Member] | LIBOR [Member]
Line of Credit Facility [Abstract]
Basis adjustment	1.25%
Unsecured Line of Credit, Third Note [Member]
Line of Credit Facility [Abstract]
Borrowing capacity	$ 7,500
Maturity date	Jun. 30, 2018
Outstanding borrowings under line of credit	$ 0	0
Unsecured Line of Credit, Third Note [Member] | LIBOR [Member]
Line of Credit Facility [Abstract]
Basis adjustment	1.25%
Unsecured Line of Credit, Fourth Note [Member]
Line of Credit Facility [Abstract]
Borrowing capacity	$ 10,000
Maturity date	Sep. 30, 2018
Outstanding borrowings under line of credit	$ 1,000	$ 0
Unsecured Line of Credit, Fourth Note [Member] | LIBOR [Member]
Line of Credit Facility [Abstract]
Basis adjustment	1.20%