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Fair Value Measurements - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Fair Value Disclosures [Abstract]        
Impairment charges $ 7.1 $ 11.9    
Estimated fair value of long-term debt 2,730.0 2,730.0 $ 3,010.0 $ 2,200.0
Long-term debt $ 2,900.0 $ 2,900.0 $ 3,150.0 $ 2,150.0