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Fair Value Measurements (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Financial Instruments        
Cash and cash equivalents $ 984,643 $ 471,901 $ 830,014 $ 740,481
Retail | Minimum        
Financial Instruments        
Fair value assumption, interest rate 0.00% 0.00%    
Retail | Maximum        
Financial Instruments        
Fair value assumption, interest rate 7.51% 7.25%    
Fair Value | Level 1        
Financial Instruments        
Cash and cash equivalents $ 984,643 $ 471,901    
Fair Value | Level 2 | Retail        
Financial Instruments        
Long-term debt 1,766,308 1,414,105    
Fair Value | Level 2 | Institutional        
Financial Instruments        
Long-term debt 501,461 513,647    
Fair Value | Level 2 | Other        
Financial Instruments        
Long-term debt 215,456 4,675    
Book Value        
Financial Instruments        
Cash and cash equivalents 984,643 471,901    
Book Value | Retail        
Financial Instruments        
Long-term debt 1,753,250 1,453,250    
Book Value | Institutional        
Financial Instruments        
Long-term debt 533,015 532,722    
Book Value | Other        
Financial Instruments        
Long-term debt $ 215,538 $ 4,749