Repurchase Agreements - Narrative (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Disclosure of Repurchase Agreements [Abstract]
Repurchase agreements	$ 301,656,523	$ 103,941,174
Fair Value of securities pledged, including accrued interest receivable	319,981,000	109,863,000
Cash Held on Deposit with Repurchase Agreement Counterparties	0	449,000
Weighted Average Borrowing Rate	0.37%	0.49%
Maximum Amount at Risk	$ 18,200,000