Investments (Investment affiliate rollforward) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Equity Method Investments [Line Items]
Investments in affiliates, beginning of period			$ 100,137
Income from investment affiliates	$ 2,178	$ 9,466	15,246	$ 14,654
Investments in affiliates, end of period	127,247		127,247
Investment affiliate
Schedule of Equity Method Investments [Line Items]
Investments in affiliates, beginning of period	113,471	94,697	100,137	100,431
Income from investment affiliates	11,598	(786)	11,864	(11,708)
Income from investment affiliates	2,178	9,466	15,246	14,654
Investments in affiliates, end of period	$ 127,247	$ 103,377	$ 127,247	$ 103,377

Total investments in (A) an entity in which the entity has significant influence, but does not have control, (B) subsidiaries that are not required to be consolidated and are accounted for using the equity and or cost method, and (C) an entity in which the reporting entity shares control of the entity with another party or group. Includes long-term advances receivable from a party that is affiliated with the reporting entity by means of direct or indirect ownership.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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