Derivative Instruments (Details 7) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Additional disclosure
Cash collateral for derivatives	$ 134,409,000	$ 54,142,000
Fair value of pledged asset as collateral against the related linked repurchase agreements	4,281,055,000	5,097,966,000
U.S. Treasury securities
Additional disclosure
Amount posted as collateral	39,800,000
Interest rate swaps
Additional disclosure
Cash collateral for derivatives	11,200,000	18,200,000
Interest rate swaps | U.S. Treasury securities
Additional disclosure
Securities received as collateral	94,400,000
Borrowings under repurchase agreement
Additional disclosure
Cash posted with one of the repurchase agreement counterparties to satisfy a margin call	1,500,000
Borrowings under repurchase agreement | U.S. Treasury securities
Additional disclosure
Amount posted as collateral	39,800,000
Linked Transactions | Borrowings under repurchase agreement
Additional disclosure
Fair value of pledged asset as collateral against the related linked repurchase agreements	$ 4,900,000