Fair Value Measurements and Interest Rate Derivatives - Fair Value Measurements (Details) $ in Thousands	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2011 item
Assets:
Interest rate derivatives	$ 2,725	$ 4,789
Former Chairman
Liabilities:
Number of installments to be paid out under the Retirement Benefit Agreement | item			10
Total amount paid to date under the Retirement Benefit Agreement	1,600
Level 2
Assets:
Life insurance policy	398	386
Total assets	3,123	5,175
Liabilities:
Retirement benefit agreement	398	386
Total liabilities	398	386
Level 2 | Interest Rate Swap
Assets:
Interest rate derivatives	2,725	4,789
Total at the end of the period
Assets:
Life insurance policy	398	386
Total assets	3,123	5,175
Liabilities:
Retirement benefit agreement	398	386
Total liabilities	398	386
Total at the end of the period | Interest Rate Swap
Assets:
Interest rate derivatives	$ 2,725	$ 4,789