Repurchase Agreements - Maturities (Details) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 318,204,000	$ 109,863,000
Repurchase agreements	308,735,338	103,941,174
Weighted Average Borrowing Rate	0.38%	0.49%
[MaturityOvernightMember]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	5,172,000	0
Repurchase agreements	4,966,000	0
Weighted Average Borrowing Rate	0.38%	0.00%
[MaturityUpTo30DaysMember]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	255,371,000	109,863,000
Repurchase agreements	247,056,000	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	57,661,000	0
Repurchase agreements	56,713,000	0
Weighted Average Borrowing Rate	0.39%	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%