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Goodwill and Intangible Assets (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill [Line Items]      
Balance at end of year $ 531,556,000 $ 531,556,000  
Goodwill impairment charges 0 0 $ 0
Subsidiaries | Six Reporting Units      
Goodwill [Line Items]      
Goodwill $ 530,600,000 530,600,000  
Indefinite-Lived Intangible Assets (Excluding Goodwill)   $ 963,000  
Goodwill impairment test fair value exceeding adjusted net book value 75.00%