Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivatives
Gross Amounts of Recognized Financial Instruments	$ 404,019	$ 243,118
Securities purchased under agreement to resell
Gross Amounts of Recognized Financial Instruments	307,872	93,633
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts Presented in the Statement of Financial Position	307,872	93,633
Financial Instruments/Collateral	(307,872)	(93,633)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	711,138	336,156
Gross Amounts Offset in the Statement of Financial Position	(377,506)	(215,066)
Net Amounts Presented in the Statement of Financial Position	333,632	121,090
Financial Instruments/Collateral	(322,917)	(100,827)
Net Amount	10,715	20,263
Derivatives
Gross Amounts of Recognized Financial Instruments	41,949	58,687
Securities sold under agreement to repurchase
Gross Amounts of Recognized Financial Instruments	0	0
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts Presented in the Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	0	0
Total Liabilities
Gross Amounts of Recognized Financial Instruments	41,949	58,687
Gross Amounts Offset in the Statement of Financial Position	(41,949)	(58,687)
Net Amounts Presented in the Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	0	0
Counterparty
Derivatives
Gross Amounts of Recognized Financial Instruments	403,266	242,523
Gross Amounts Offset in the Statement of Financial Position	(377,506)	(215,066)
Net Amounts Presented in the Statement of Financial Position	25,760	27,457
Financial Instruments/Collateral	(15,045)	(7,194)
Net Amount	10,715	20,263
Derivatives
Gross Amounts of Recognized Financial Instruments	41,949	58,687
Gross Amounts Offset in the Statement of Financial Position	(41,949)	(58,687)
Net Amounts Presented in the Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	$ 0	$ 0