Commitments and Contingencies, Variable Interest Entities (Details) - Low-Income Housing Limited Partnerships - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amount Reflected in Consolidated Balance Sheets [Abstract]
Current assets	$ 426	$ 375
Property, plant and equipment, net	1,882	2,025
Other noncurrent assets	137	125
Total assets	2,445	2,525
Current liabilities	1,214	1,269
Mortgages and other long-term debt payable	486	486
Other noncurrent liabilities	56	54
Total liabilities	$ 1,756	$ 1,809