Derivative Instruments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Total outstanding derivative contracts	$ 381,393	$ 390,924
Outstanding derivative contracts maturity, maximum	1 year 3 months 0 days
Cash Flow Hedges [Member]
Derivative [Line Items]
Total outstanding derivative contracts	208,446	165,000
Balance Sheet Hedges [Member]
Derivative [Line Items]
Total outstanding derivative contracts	$ 172,947	$ 225,924