Balance Sheet Offsetting (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financial assets
Net Amount Presented in the Balance Sheet	$ 59,055
Reverse repurchase agreements
Gross Amount Recognized	200,000
Net Amount Presented in the Balance Sheet	200,000
Gross Amounts Not Offset in the Balance Sheet, Securities Collateral	(200,000)
Derivatives designated as hedging instruments
Financial assets
Gross Amount Recognized		2,300
Financial liabilities
Gross Amount Recognized		100
Derivatives designated as hedging instruments | Master netting arrangements and similar agreements
Financial assets
Gross Amount Recognized		2,249
Gross Amount Offset		(83)
Net Amount Presented in the Balance Sheet		2,166
Gross Amounts Not Offset in the Balance Sheet, Securities Collateral		(1,034)
Net Amount		1,132
Financial liabilities
Gross Amount Recognized		83
Gross Amount Offset		(83)
Derivatives not designated as hedging instruments
Financial assets
Gross Amount Recognized	61,200	66,800
Financial liabilities
Gross Amount Recognized	60,000	65,900
Derivatives not designated as hedging instruments | Master netting arrangements and similar agreements
Financial assets
Gross Amount Recognized	2,867	1,920
Gross Amount Offset	(2,153)	(1,454)
Net Amount Presented in the Balance Sheet	714	466
Net Amount	714	466
Financial liabilities
Gross Amount Recognized	59,505	65,461
Gross Amount Offset	(2,153)	(1,453)
Net Amount Presented in the Balance Sheet	57,352	64,008
Gross Amounts Not Offset in the Balance Sheet, Securities Collateral	(40,102)	(48,697)
Net Amount	17,250	15,311
Total financial assets and total financial liabilities
Financial liabilities
Gross Amount Recognized	59,505	65,544
Total financial assets and total financial liabilities | Master netting arrangements and similar agreements
Financial assets
Gross Amount Recognized	202,867	4,169
Gross Amount Offset	(2,153)	(1,537)
Net Amount Presented in the Balance Sheet	200,714	2,632
Gross Amounts Not Offset in the Balance Sheet, Securities Collateral	(200,000)	(1,034)
Net Amount	714	1,598
Financial liabilities
Gross Amount Recognized	59,505	65,544
Gross Amount Offset	(2,153)	(1,536)
Net Amount Presented in the Balance Sheet	57,352	64,008
Gross Amounts Not Offset in the Balance Sheet, Securities Collateral	(40,102)	(48,697)
Net Amount	$ 17,250	$ 15,311