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Debt (Details Textual) (USD $)
9 Months Ended 12 Months Ended
Sep. 30, 2014
Dec. 31, 2013
Current Unsecured Bank Credit Facility [Member]
   
Debt (Textual) [Abstract]    
Unsecured borrowings $ 150,000  
Interest rate variable spread percentage 1.15%  
Variable commitment fee unused portion 0.15%  
Line of Credit Facility, Expiration Date Oct. 06, 2015  
Line of Credit Facility, Interest Rate at Period End 1.30%  
Previous Unsecured Bank Credit Facility [Member]
   
Debt (Textual) [Abstract]    
Line of Credit Facility, Expiration Date Dec. 19, 2011  
Senior Notes 2015 [Member]
   
Debt (Textual) [Abstract]    
Senior Notes 6.05%  
Senior Notes 2015 [Member] | Senior Note 6.05 % [Member]
   
Debt (Textual) [Abstract]    
Unaccrued discount 18 38
Aggregate principal amount 75,000 75,000
Maturity date of Senior note Jun. 15, 2015 Jun. 15, 2015
Imputed rate 6.10% 6.10%
Senior Notes 2016 [Member]
   
Debt (Textual) [Abstract]    
Senior Notes 6.85%  
Senior Notes 2016 [Member] | Senior Note 6.85 % [Member]
   
Debt (Textual) [Abstract]    
Unaccrued discount 59 88
Aggregate principal amount $ 125,000 $ 125,000
Maturity date of Senior note Apr. 15, 2016 Apr. 15, 2016
Imputed rate 6.90% 6.90%
HMEC's Bank Credit Facility [Member] | Current Unsecured Bank Credit Facility [Member]
   
Debt (Textual) [Abstract]    
Line of Credit Facility, Expiration Date Jul. 30, 2019