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Earnings Per Share (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
Earnings Per Share [Abstract]      
Anti-dilutive stock-based compensation awards that were excluded from the calculation of diluted earnings per share (in shares) 11,940 0  
Earnings per share numerator:      
Net income attributable to common shareholders before allocation of earnings to participating securities $ 67,835 $ 66,547 $ 239,593
Earnings allocated to participating securities 61 0  
Net income attributable to common shareholders after allocation of earnings to participating securities $ 67,774 $ 66,547  
Earnings per share denominator:      
Weighted-average number of shares of common stock outstanding - basic and diluted (in shares) 13,376,000 13,405,000  
Earnings per share - basic and diluted (in dollars per share) $ 5.07 $ 4.96