Annual report pursuant to Section 13 and 15(d)

Derivative Instruments and Hedging Activities Interest Spread on Interest Rate Swaps (Details)

v3.8.0.1
Derivative Instruments and Hedging Activities Interest Spread on Interest Rate Swaps (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]      
Average Notional Amount $ 19,953,655,000 $ 21,641,836,000  
Interest Rate Swap [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Interest Expense on Interest Rate Swap and Swaption Agreements 8,823,000 21,074,000 $ 85,636,000
Net Long Position [Member] | Interest Rate Swap [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Average Notional Amount $ 19,447,067,044 $ 15,506,763,268 $ 16,091,714,468