Derivative Instruments (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Notional Amount of Derivatives, Total [Abstract]
Total outstanding derivative contracts	$ 189,972	$ 390,924
Outstanding derivative contracts maturity, maximum	1 year 3 months 0 days
Cash Flow Hedges [Member]
Notional Amount of Derivatives, Total [Abstract]
Total outstanding derivative contracts	112,632	165,000
Balance Sheet Hedges [Member]
Notional Amount of Derivatives, Total [Abstract]
Total outstanding derivative contracts	$ 77,340	$ 225,924