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Short-Term Borrowings (Details) - USD ($)
1 Months Ended
Apr. 23, 2015
Oct. 31, 2015
Apr. 30, 2015
Mar. 31, 2016
Dec. 31, 2015
Short-term Debt [Line Items]          
Short-term borrowings       $ 311,885,000 $ 310,659,000
Interest rates on short-term borrowings outstanding       1.80%  
Intraday Credit Facility [Member] | Revolving Credit Facility [Member]          
Short-term Debt [Line Items]          
Credit facility maximum amount     $ 500,000,000.0    
Credit facility term     6 months    
Debt instrument extension term     6 months    
Intraday Credit Facility [Member] | Revolving Credit Facility [Member] | Prime Rate [Member]          
Short-term Debt [Line Items]          
Debt instrument, basic spread on variable rate 0.50%        
Amended Intraday Credit Facility [Member] | Revolving Credit Facility [Member]          
Short-term Debt [Line Items]          
Credit facility maximum amount   $ 300,000,000.0      
Credit facility, extension term   364 days      
Loan Facility [Member] | Revolving Credit Facility [Member]          
Short-term Debt [Line Items]          
Credit facility maximum amount   $ 50,000,000.0      
Loan Facility [Member] | Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Short-term Debt [Line Items]          
Debt instrument, basic spread on variable rate   3.75%