Corporate Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of unrecognized tax benefits excluding amounts pertaining to examined tax returns RollForward
Beginning balance	$ 1,187	$ 1,187	$ 1,187
Gross decreases for tax positions taken in prior years	0	0	0
Gross increases for tax positions taken in prior years	0	0	0
Decreases relating to settlements with taxing authorities	0	0	0
Reductions due to a lapse of applicable statute of limitations	(32)	0	0
Ending balance	$ 1,155	$ 1,187	$ 1,187