Derivative Financial Instruments (Details 6) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Credit Risk
Number of clearing brokers, primarily utilized	3
Cash Flow Hedges
Fair values of derivative financial instruments
Asset Derivatives	$ 135,763	$ 205,342
Liability Derivatives	54,921	65,203
Cash Flow Hedges | Exchange-Traded Derivatives | Broker margin deposits
Fair values of derivative financial instruments
Asset Derivatives	78,249	121,490
Cash Flow Hedges | Forward derivative contracts
Fair values of derivative financial instruments
Asset Derivatives	57,470	83,826
Liability Derivatives	48,272	58,507
Cash Flow Hedges | Open Forward Exchange Contracts | Other assets
Fair values of derivative financial instruments
Asset Derivatives	27	9
Cash Flow Hedges | Interest rate cap | Other assets
Fair values of derivative financial instruments
Asset Derivatives	17	17
Cash Flow Hedges | Interest rate swaps | Other long-term liabilities
Fair values of derivative financial instruments
Liability Derivatives	6,649	6,696
Derivatives designated as hedging instruments and firm commitments | Cash Flow Hedges
Fair values of derivative financial instruments
Asset Derivatives	17,895	30,600
Derivatives designated as hedging instruments and firm commitments | Cash Flow Hedges | Exchange-Traded Derivatives | Broker margin deposits
Fair values of derivative financial instruments
Asset Derivatives	17,895	30,600
Derivatives not designated as hedging instruments | Cash Flow Hedges
Fair values of derivative financial instruments
Asset Derivatives	117,868	174,742
Liability Derivatives	54,921	65,203
Derivatives not designated as hedging instruments | Cash Flow Hedges | Exchange-Traded Derivatives | Broker margin deposits
Fair values of derivative financial instruments
Asset Derivatives	60,354	90,890
Derivatives not designated as hedging instruments | Cash Flow Hedges | Forward derivative contracts
Fair values of derivative financial instruments
Asset Derivatives	57,470	83,826
Liability Derivatives	48,272	58,507
Derivatives not designated as hedging instruments | Cash Flow Hedges | Open Forward Exchange Contracts | Other assets
Fair values of derivative financial instruments
Asset Derivatives	27	9
Derivatives not designated as hedging instruments | Cash Flow Hedges | Interest rate cap | Other assets
Fair values of derivative financial instruments
Asset Derivatives	17	17
Derivatives not designated as hedging instruments | Cash Flow Hedges | Interest rate swaps | Other long-term liabilities
Fair values of derivative financial instruments
Liability Derivatives	$ 6,649	$ 6,696