Fair Value Measurements and Interest Rate Derivatives - Fair Value Measurements (Details) $ in Thousands		3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2011 item
Fair value of assets and liabilities measured at fair value on a recurring and nonrecurring basis
Impairment loss		$ 1,394	$ 1,394
Assets:
Interest rate derivative assets	$ 32			$ 4,789
Liabilities:
Interest rate derivative liabilities	885
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	430			5,175
Liabilities:
Retirement benefit agreement	398			386
Total liabilities	1,283			386
Level 2 | Interest Rate Swap
Assets:
Interest rate derivative assets	32			4,789
Liabilities:
Interest rate derivative liabilities	885
Total at the end of the period
Assets:
Life insurance policy	398			386
Total assets	430			5,175
Liabilities:
Retirement benefit agreement	398			386
Total liabilities	1,283			386
Total at the end of the period | Interest Rate Swap
Assets:
Interest rate derivative assets	32			$ 4,789
Liabilities:
Interest rate derivative liabilities	$ 885