Investments (Investment affiliate rollforward) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investments in affiliates, beginning of period	$ 100,137
Income from investment affiliates	13,068	$ 5,188
Investments in affiliates, end of period	113,471
Investment affiliate
Schedule of Equity Method Investments [Line Items]
Investments in affiliates, beginning of period	100,137	100,431
Net capital (distributions) contributions	266	(10,922)
Income from investment affiliates	13,068	5,188
Investments in affiliates, end of period	$ 113,471	$ 94,697

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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