Debt Interest Rate Swaps (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Incremental interest expense arising from difference between interest paid and interest received related to swap agreements	$ 1,691	$ 1,308	$ 1,388
Interest Rate Swap 1, Credit Facility [Member]
Derivative [Line Items]
Interest rate swap agreement, fixed interest rate	1.185%
Interest rate swap agreement, notional amount	$ 25,000
Interest Rate Swap 2, Credit Facility [Member]
Derivative [Line Items]
Interest rate swap agreement, fixed interest rate	1.185%
Interest rate swap agreement, notional amount	$ 25,000
Interest Rate Swap 3, Credit Facility Member
Derivative [Line Items]
Interest rate swap agreement, fixed interest rate	1.10%
Interest rate swap agreement, notional amount	$ 50,000
Interest Rate Swap 4, Credit Facility [Member]
Derivative [Line Items]
Interest rate swap agreement, fixed interest rate	1.05%
Interest rate swap agreement, notional amount	$ 25,000
Interest Rate Swap 5, Credit Facility [Member]
Derivative [Line Items]
Interest rate swap agreement, fixed interest rate	0.99%
Interest rate swap agreement, notional amount	$ 25,000