Fair Value of Financial Assets and Liabilities, Financial Impact of Qualifying Cash Flow Hedges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive loss related to cash flow hedges at Jan. 1	$ (133)	$ (209)	$ (285)
After-tax net realized losses on derivative transactions reclassified into earnings	76	76	76
Accumulated other comprehensive loss related to cash flow hedges at Dec. 31	$ (57)	$ (133)	$ (209)