Investments in Affiliates - Equity Method Investment Summarized Balance Sheet Information (Details) - Joint Venture Unconsolidated - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Investments In And Advances To Affiliates [Line Items]
Equity Method Investment, Summarized Financial Information, Assets	$ 123,562	$ 160,359	$ 150,637
Equity Method Investments Summarized Financial Information Net Assets	69,637	98,592	93,675
Other Affiliates [Member]
Investments In And Advances To Affiliates [Line Items]
Equity Method Investment, Summarized Financial Information, Assets	123,600
Equity Method Investments	$ 38,682	$ 38,469	$ 36,410