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Acquisitions and Disposals - New Energy Risk narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Jul. 24, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Business Acquisition [Line Items]        
Goodwill, acquired during period   $ 14,084 $ 807,381  
New Energy Risk Inc. [Member]        
Business Acquisition [Line Items]        
Business acquisition, percentage of voting interests acquired 63.63%      
Business combination, step acquisition, equity interest in acquiree, percentage 31.16%      
Business combination, step acquisition, equity interest in acquiree, including subsequent acquisition, percentage 94.79%      
Cash consideration $ 8,800      
Business combination, step acquisition, equity interest in acquiree, remeasurement gain 2,500      
Goodwill, acquired during period $ 13,400      
XL Innovate Fund [Member]        
Business Acquisition [Line Items]        
Noncontrolling interest, ownership percentage by noncontrolling owners 5.21%     5.21%