Summary of Significant Accounting Policies - Additional Information (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Subsidiary or Equity Method Investee, Cumulative Percentage Ownership after All Transactions	83.68%
Noncontrolling interest	$ 361,000	$ 2,631,000
Cash equivalents maximum maturity period of highly liquid investments	90 days
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of business days for related transactions	1 day
Loan forgiven over service period	3 years
Minimum [Member] | Furniture and Fixtures [Member]
Summary Of Significant Accounting Policies [Line Items]
Depreciation and amortization on straight-line basis	3 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of business days for related transactions	2 days
Loan forgiven over service period	5 years
Maximum [Member] | Furniture and Fixtures [Member]
Summary Of Significant Accounting Policies [Line Items]
Depreciation and amortization on straight-line basis	7 years
Securities
Summary Of Significant Accounting Policies [Line Items]
Number of business days for related transactions	2 days
Commodities
Summary Of Significant Accounting Policies [Line Items]
Number of business days for related transactions	1 day