Summary of Significant Accounting Policies - The Company (Details) a in Thousands	12 Months Ended
	Dec. 31, 2017 a segment variable_interest_entity mi	Dec. 31, 2016 variable_interest_entity
Revenues from External Customers and Long-Lived Assets [Line Items]
Variable Interest Entity, Number of Entities Consolidated | variable_interest_entity	1	1
Business segments | segment	5
Area of land | a	270
Los Angeles, California
Revenues from External Customers and Long-Lived Assets [Line Items]
Distance from city	60
Bakersfield, California
Revenues from External Customers and Long-Lived Assets [Line Items]
Distance from city	15