Derivative Instruments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) instrument	Dec. 31, 2015 instrument
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Number of Instruments Held | instrument	2	2
Derivative, Notional Amount	$ 50
Derivative, Amount of Hedged Item	$ 50
Interest Rate Swap No. 2 [Member]
Derivatives, Fair Value [Line Items]
Derivative, Maturity Date	Aug. 29, 2019
Derivative, Notional Amount	$ 25
Derivative, Fixed Interest Rate	1.73%
Interest Rate Swap No. 1 [Member]
Derivatives, Fair Value [Line Items]
Derivative, Maturity Date	Aug. 29, 2024
Derivative, Notional Amount	$ 25
Derivative, Fixed Interest Rate	2.50%
Fair Value, Inputs, Level 2 [Member] | Interest Rate Swap No. 2 [Member]
Derivatives, Fair Value [Line Items]
Derivative, Term of Contract	5 years
Fair Value, Inputs, Level 2 [Member] | Interest Rate Swap No. 1 [Member]
Derivatives, Fair Value [Line Items]
Derivative, Term of Contract	10 years