Repurchase Agreements - Maturities (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 319,981,000	$ 109,863,000
Repurchase agreements	301,656,523	103,941,174
Weighted Average Borrowing Rate	0.37%	0.49%
[MaturityOvernightMember]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	25,386,000	0
Repurchase agreements	25,067,000	0
Weighted Average Borrowing Rate	0.35%	0.00%
[MaturityUpTo30DaysMember]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	192,889,000	109,863,000
Repurchase agreements	181,002,000	103,941,000
Weighted Average Borrowing Rate	0.38%	0.49%
[Maturity30To90DaysMember]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	101,706,000	0
Repurchase agreements	95,588,000	0
Weighted Average Borrowing Rate	0.34%	0.00%
[MaturityOver90DaysMember]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	0	0
Repurchase agreements	$ 0	$ 0
Weighted Average Borrowing Rate	0.00%	0.00%