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Fair Value Measurements - Additional Information (Detail)
shares in Millions, $ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2015
shares
Jun. 30, 2015
USD ($)
Note
Dec. 31, 2014
USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Number of notes receivable | Note   2  
$350 Million Senior Notes at 5% Interest [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Face amount of outstanding debt   $ 350.0 $ 350.0
Interest rate of Senior Notes   5.00% 5.00%
Fair value of Senior Notes   $ 351.8  
$400 Million 5% Senior Notes [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Face amount of outstanding debt   $ 400.0  
Interest rate of Senior Notes   5.00%  
Fair value of Senior Notes   $ 394.0  
3.75% Convertible Senior Notes [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Common stock warrants repurchased | shares 4.7    
Bonds A Series [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Notes receivables, carrying value   $ 87.0  
Maturity date   Jul. 01, 2034  
Bonds B Series [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Notes receivables, carrying value   $ 65.6  
Notes receivable, fair value   $ 54.0  
Maturity date   Sep. 01, 2037