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Equity (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
day
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
shares
Feb. 26, 2018
USD ($)
Dec. 26, 2013
USD ($)
Class of Stock [Line Items]          
Common stock, par value (in dollars per share) | $ / shares $ 0.001 $ 0.001      
Common stock authorized (shares) 250,000,000        
Common stock authorized for repurchase | $ $ 68,800,000       $ 350,000,000
Stock repurchased during period (shares) 5,700,000 0 6,500,000    
Stock repurchased during period | $ $ 50,000,000 $ 0 $ 85,900,000    
Preferred shares authorized (shares) 4,000,000.0        
Preferred stock issued (shares) 0 0      
Preferred shares outstanding (shares) 0 0      
Series A Preferred Stock [Member]          
Class of Stock [Line Items]          
Series A preferred shares authorized (shares) 1,000,000.0        
Series A preferred shares par value (in usd per share) | $ / shares $ 0.001        
Series A preferred shares issued (shares) 200,000        
Series A preferred shares stated value per share (in usd per share) | $ / shares $ 1,000        
Series A preferred shares dividend rate (percent) 6.00%        
Series A preferred shares accrued dividends | $ $ 3,000,000        
Series A preferred shares conversion price (in usd per share) | $ / shares $ 14.50        
Series A preferred shares, minimum stock price threshold for conversion (in usd per share) | $ / shares $ 29.00        
Series A preferred shares, number of conservative trading days | day 20        
Series A preferred shares, number of shares issuable if converted (shares) 13,793,100        
Series A preferred shares percentage of stated redemption value (percent) 100.00%        
Series A Preferred Stock [Member] | Blackstone [Member]          
Class of Stock [Line Items]          
Series A preferred shares percentage of stated redemption value (percent) 101.00%        
Series A preferred shares redemption value | $ $ 200,000,000        
If Dividends not Paid Timely [Member] | Series A Preferred Stock [Member]          
Class of Stock [Line Items]          
Series A preferred shares dividend rate (percent) 8.00%        
Subsequent Event [Member]          
Class of Stock [Line Items]          
Common stock authorized for repurchase | $       $ 500,000,000.0