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Equity Distribution Program (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Class of Stock [Line Items]      
Proceeds from issuance of common stock $ 9,700,000 $ 41,900,000 $ 465,800,000
Equity Distribution Program      
Class of Stock [Line Items]      
Stock program, maximum amount authorized     $ 500,000,000
Proceeds from issuance of common stock $ 0