Derivative Instruments and Hedging Activities Interest Spread on Interest Rate Swaps (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Average Notional Amount	$ 18,317,731,000	$ 20,642,679,000	$ 17,799,927,000	$ 22,701,140,000
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Interest Expense on Interest Rate Swap and Swaption Agreements	$ 389,000	$ 4,294,000	$ 10,867,000	$ 18,140,000