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Equity-Based Plans (Details) - USD ($)
 3 Months Ended 12 Months Ended
Jun. 30, 2015
Sep. 30, 2013
Jun. 30, 2013
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2013
stock offering 10,400,000   11,100,000       700,000
Proceeds from Issuance of Common Stock $ 271,000,000 $ 20,000,000   $ 0 $ 0 $ 271,000,000  
employee stock purchase plan shares authorized       625,000      
Employee Stock Purchase Plan, Base Pay Threshold       10.00%      
Employee Stock Purchase Plan, Purchased Stock Value Limitation       $ 25,000      
Employee Stock Purchase Plan, Share Purchase Limitation       1,500      
Employee Stock Purchase Plan, Purchase Price       95.00%      
Employee Stock Purchase Plan, Number of Available Shares       339,542      
Dividend Reinvestment and Direct Stock Purchase Plan Shares       2,500,000      
Dividend Reinvestment and Direct Stock Purchase Plan, Number of Available Shares       2,470,052