Derivative Instruments and Hedging Activities (Details)	12 Months Ended
	Dec. 31, 2014 USD ($) interest_rate_swap	Sep. 30, 2018 USD ($) interest_rate_swap	Dec. 31, 2017 USD ($)
Derivatives, Fair Value [Line Items]
Number of interest rate swaps | interest_rate_swap	2	2
Long-term debt, percentage bearing fixed interest, amount		$ 200,000,000
Long-term debt, percentage bearing variable interest, amount		98,000,000
5 year interest rate swap
Derivatives, Fair Value [Line Items]
Notional amount	$ 25,000,000
Fixed interest rate	2.50%
Maturity date	Aug. 29, 2024
Interest rate derivative assets		196,000	$ 70,000
10 year interest rate swap
Derivatives, Fair Value [Line Items]
Notional amount	$ 25,000,000
Fixed interest rate	1.73%
Maturity date	Aug. 29, 2019
Interest rate derivative assets		562,000
Interest rate derivative liabilities			(492,000)
Interest rate swap
Derivatives, Fair Value [Line Items]
Notional amount	$ 50,000,000	50,000,000
Interest rate derivative assets		757,000
Interest rate derivative liabilities			(422,000)
Level 2 | 5 year interest rate swap
Derivatives, Fair Value [Line Items]
Interest rate derivative assets			70,000
Level 2 | 10 year interest rate swap
Derivatives, Fair Value [Line Items]
Interest rate derivative liabilities			(492,000)
Other noncurrent assets | Level 2 | 5 year interest rate swap
Derivatives, Fair Value [Line Items]
Interest rate derivative assets		196,000	70,000
Other noncurrent assets | Level 2 | 10 year interest rate swap
Derivatives, Fair Value [Line Items]
Interest rate derivative assets		$ 561,000
Other noncurrent liabilities | Level 2 | 10 year interest rate swap
Derivatives, Fair Value [Line Items]
Interest rate derivative liabilities			$ (492,000)
London Interbank Offered Rate (LIBOR) | Minimum
Derivatives, Fair Value [Line Items]
Derivative, basis spread on variable rate		1.375%
London Interbank Offered Rate (LIBOR) | Maximum
Derivatives, Fair Value [Line Items]
Derivative, basis spread on variable rate		2.00%