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Equity (Stock Repurchase Program) (Details) - USD ($)
$ / shares in Units, shares in Millions
 1 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Equity [Abstract]        
Repurchase of common stock, authorized amount   $ 50,000,000.0    
Stock repurchased and retired during period, shares     10.2 5.7
Common stock value acquired including acquisition costs     $ 98,400,000 $ 70,400,000
Average cost per share $ 11.51   $ 9.67 $ 12.25
Class of Stock [Line Items]        
Shares repurchased during period 4.0      
Repurchase of stock $ 45,900,000 $ 45,928,000 $ 98,429,000 $ 70,416,000
3.125% senior convertible notes        
Class of Stock [Line Items]        
Stated interest rate (as a percent) 3.125% 3.125%