Fair Value Disclosures - Financial instruments measured at fair value on non-recurring basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Assets held for sale	$ 4,032	$ 5,385
Fair value on a non-recurring basis
Assets:
Assets held for sale	4,032	5,385
Fair value on a non-recurring basis | Level 1
Assets:
Assets held for sale	0
Fair value on a non-recurring basis | Level 2
Assets:
Assets held for sale	4,032	$ 5,385
Fair value on a non-recurring basis | Level 3
Assets:
Assets held for sale	$ 0