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Shareholders' Equity - Narrative (Details) - USD ($)
1 Months Ended 3 Months Ended 6 Months Ended
Mar. 15, 2017
May 15, 2016
Mar. 17, 2016
Mar. 31, 2017
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Class of Stock [Line Items]                
Excess fair value of consideration paid over carrying value of preferred shares         $ 0 $ (9,609,000) $ 0 $ (9,609,000)
Common Shares                
Class of Stock [Line Items]                
Stock repurchase program, authorized amount (up to)     $ 150,000,000          
Stock repurchase program, period in force 12 months   12 months          
Stock repurchase program, amount not utilized       $ 106,600,000        
Stock repurchase program, additional authorized amount (up to) $ 150,000,000              
Number of shares surrendered to satisfy tax withholding obligations (in shares)             6,694  
Series E                
Class of Stock [Line Items]                
Stock redeemed (in shares)   11,000,000            
Redemption price per share (in dollars per share)   $ 25.00            
Preferred stock, redemption amount   $ 275,000,000            
Excess fair value of consideration paid over carrying value of preferred shares           $ 9,600,000   $ 9,609,000