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Long-term Debt (Details) (USD $)
In Thousands, unless otherwise specified
1 Months Ended 12 Months Ended
Nov. 30, 2011
Dec. 31, 2014
Dec. 31, 2013
Line of Credit Facility [Line Items]      
Weighted average interest rate of long-term bank debt (in hundredths)   2.40%us-gaap_LongtermDebtWeightedAverageInterestRate 2.30%us-gaap_LongtermDebtWeightedAverageInterestRate
Syndicated Senior Revolving Credit Facility [Member]      
Line of Credit Facility [Line Items]      
Borrowing capacity $ 250,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
   
Term of revolving credit facility 5 years    
Expiration date of revolving credit facility Nov. 30, 2016    
Variable rate of credit facility   LIBOR  
Outstanding credit facility   $ 60,000us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
$ 79,250us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
Syndicated Senior Revolving Credit Facility [Member] | Minimum [Member]      
Line of Credit Facility [Line Items]      
Basis spread (in hundredths)   1.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
 
Syndicated Senior Revolving Credit Facility [Member] | Maximum [Member]      
Line of Credit Facility [Line Items]      
Basis spread (in hundredths)   2.10%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember