Summary of Offsetting Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Derivative instruments (assets)
Gross fair value of recognized assets	$ 25,952	[1],[2]	$ 25,250	[3],[4]
Financial instruments	(25,618)	[1]	(25,023)	[3]
Cash collateral received	78	[1]	0	[3]
Net fair value	256	[1]	227	[3]
Short-term reverse repurchase agreements
Gross fair value of recognized assets	750,000	[2],[5]
Financial instruments	(750,000)	[5]
Cash collateral received	0	[5]
Net fair value	0	[5]
Financial instruments (assets)
Gross fair value of recognized assets	775,952	[2]
Financial instruments	(775,618)
Cash collateral received	78
Net fair value	256
Derivatives instrument (liabilities)
Gross fair value of recognized assets/liabilities	157,673	[2],[6]	171,387	[4],[7]
Financial Instruments	(25,618)	[6]	(25,023)	[7]
Cash collateral received/(pledged)	(129,344)	[6]	(143,540)	[7]
Net fair value	$ 2,711	[6]	$ 2,824	[7]

[1] The estimated fair value of derivative instrument assets is reported in other assets in the condensed consolidated balance sheets. Derivative transactions entered into under ISDA master agreements include income and expense accruals.

[2] The gross fair value of derivative instrument and short-term reverse repurchase agreement assets are not netted against offsetting liabilities for presentation on the condensed consolidated balance sheets.

[3] The estimated fair value of derivative instrument assets is reported in other assets in the condensed consolidated balance sheets and includes income and expense accruals.

[4] The gross fair value of derivative instrument assets is not netted against offsetting liabilities for presentation on the condensed consolidated balance sheets.

[5] The estimated fair value of short-term reverse repurchase agreement assets is reported in short-term investments in the condensed consolidated balance sheets. The collateral is held by an independent third-party custodian under a tri-party agreement.

[6] The estimated fair value of derivative instrument liabilities is reported in other liabilities in the condensed consolidated balance sheets. Derivative transactions entered into under ISDA master agreements include income and expense accruals.

[7] The estimated fair value of derivative instrument liabilities is reported in other liabilities in the condensed consolidated balance sheets and includes income and expense accruals.