Fair Value Measurement - Narrative (Details)	6 Months Ended
	Jun. 30, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Discount for selling costs	10.00%
Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Period when new appraisals are received	28 days
Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Period when new appraisals are received	42 days