Investments in Affiliates - Investments in Affiliates (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Investments In And Advances To Affiliates [Line Items]
Investments in affiliates	$ 99,495	$ 38,469	$ 37,170
Joint Venture Unconsolidated [Member]
Investments In And Advances To Affiliates [Line Items]
Investments in affiliates	99,495	38,469	37,170
Joint Venture Unconsolidated [Member] | Foreign Affiliates [Member]
Investments In And Advances To Affiliates [Line Items]
Investments in affiliates	63,000	0	0
Joint Venture Unconsolidated [Member] | Equity Method investments in Real Estate Affiliates [Member]
Investments In And Advances To Affiliates [Line Items]
Investments in affiliates	27,591	29,472	27,329
Joint Venture Unconsolidated [Member] | Equity Method Investment in Asphalt Terminal Affiliate [Member]
Investments In And Advances To Affiliates [Line Items]
Investments in affiliates	$ 8,904	$ 8,997	$ 9,841