Commitments and Contingencies, Letters of Credit (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 50,000,000
Letters of Credit Outstanding, Amount	9,400,000	$ 9,400,000
Purchase Commitment, Property Value	205,000,000
Purchase Commitment, Remaining Minimum Amount Committed	$ 62,000,000