Derivative Financial Instruments Schedule of Derivative Assets and Liabilities by Counterparty (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	$ 569,691
Derivative Liabilities	48,435
JP Morgan Chase Counterparty [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	119,358
Derivative Liabilities	15,217
Citibank, N.A. Counterparty [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	87,788
Derivative Liabilities	438
Barclays Capital Counterparty [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	53,131
Derivative Liabilities	0
BMO Financial Group [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	42,192
Derivative Liabilities	0
BNP Paribas [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	40,677
Derivative Liabilities	0
J. Aron & Company [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	33,071
Derivative Liabilities	0
Wells Fargo Bank, N.A. Counterparty [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	31,419
Derivative Liabilities	20,981
Credit Suisse [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	29,795
Derivative Liabilities	0
Toronto Dominion Counterparty [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	29,206
Derivative Liabilities	0
Societe Generale [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	28,179
Derivative Liabilities	2,177
Merrill Lynch [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	24,356
Derivative Liabilities	0
Merrill Lynch [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	16,880
Derivative Liabilities	2,775
Morgan Stanley [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	16,184
Derivative Liabilities	885
Den Norske Bank [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	9,790
Derivative Liabilities	0
BP Corporation North America [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	4,040
Derivative Liabilities	0
Deutsche Bank [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	2,194
Derivative Liabilities	2,363
Vitol [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	1,345
Derivative Liabilities	0
Macquarie Bank [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	86
Derivative Liabilities	0
Royal Bank of Canada [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	0
Derivative Liabilities	262
UBS [Member]
Schedule of Derivative Assets and Liabilities by Counterparty [Line Items]
Derivative Assets	0
Derivative Liabilities	$ 3,337