Volume of Activity for Interest Rate Derivative Instruments (Details) (Interest Rate Swap, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Interest Rate Swap
Derivative [Line Items]
Notional Value, Beginning of period	$ 10,500,000	$ 10,700,000
Additions		800,000
Expirations and terminations	(2,200,000)	(1,000,000)
Notional Value, End of period	$ 8,300,000	$ 10,500,000