Summary of Significant Accounting Policies - Notes Receivable (Textual) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan maturity term	3 years
Note receivable	$ 3.5	$ 3.4
Note Receivable Year One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan bears interest rate	6.30%
Note Receivable Year Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan bears interest rate	7.00%
Note Receivable Year Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan bears interest rate	8.00%