Balance Sheet Offsetting (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Reverse repurchase agreements
Gross Amount Recognized	$ 700,555	$ 700,149
Gross amount Offset	(500,253)	(500,041)
Net Amount Presented in the Balance Sheet	200,302	200,108
Gross Amounts Not Offset in the Balance Sheet, Securities Collateral	(200,000)	(200,000)
Net Amount	302	108
Repurchase agreements
Gross Amount Recognized	500,253	500,041
Gross Amount Offset	(500,253)	(500,041)
Derivatives not designated as hedging instruments | Swap
Financial instruments that are eligible for offset
Principal amount of collateral swap agreement	500,000
Financial assets
Fair Value of Securities Pledged as Collateral	534,200
Financial liabilities
Fair Value of Securities Received as Collateral	521,000
Derivatives not designated as hedging instruments | Master netting arrangements and similar agreements
Financial assets
Gross Amount Recognized	8,661	7,669
Gross Amount Offset	(207)	(1,562)
Net Amount Presented in the Balance Sheet	8,454	6,107
Net Amount	8,454	6,107
Financial liabilities
Gross Amount Recognized	81,776	51,125
Gross Amount Offset	(207)	(1,562)
Net Amount Presented in the Balance Sheet	81,569	49,563
Gross Amounts Not Offset in the Balance Sheet, Securities Collateral	(16,697)	(14,654)
Gross Amounts Not Offset in the Balance Sheet, Cash Collateral	(57,162)	(28,046)
Net Amount	7,710	6,863
Derivatives designated and not designated as hedging instruments | Master netting arrangements and similar agreements
Financial assets
Gross Amount Recognized	709,216	707,818
Gross Amount Offset	(500,460)	(501,603)
Net Amount Presented in the Balance Sheet	208,756	206,215
Gross Amounts Not Offset in the Balance Sheet, Securities Collateral	(200,000)	(200,000)
Net Amount	8,756	6,215
Financial Liabilities
Gross Amount Recognized	582,029	551,166
Gross Amount Offset	(500,460)	(501,603)
Net Amount Presented in the Balance Sheet	81,569	49,563
Gross Amounts Not Offset in the Balance Sheet, Securities Collateral	(16,697)	(14,654)
Gross Amounts Not Offset in the Balance Sheet, Cash Collateral	(57,162)	(28,046)
Net Amount	$ 7,710	$ 6,863