Repurchase Agreements (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Credit Suisse
Repurchase agreements counterparty risk
Amount at Risk	$ 646,190
Weighted Average Months to Maturity for Repurchase Agreements	1 month
Counterparty risk as percent of Stockholders' Equity, minimum	19.50%
Wells Fargo
Repurchase agreements counterparty risk
Amount at Risk	321,159
Weighted Average Months to Maturity for Repurchase Agreements	14 months
Counterparty risk as percent of Stockholders' Equity, minimum	9.70%
Wells Fargo Bank
Repurchase agreements counterparty risk
Amount at Risk	193,300
Wells Fargo Securities LLC
Repurchase agreements counterparty risk
Amount at Risk	127,800
UBS
Repurchase agreements counterparty risk
Amount at Risk	269,431
Weighted Average Months to Maturity for Repurchase Agreements	29 months
Counterparty risk as percent of Stockholders' Equity, minimum	8.10%
UBS AG
Repurchase agreements counterparty risk
Amount at Risk	259,200
UBS Securities LLC
Repurchase agreements counterparty risk
Amount at Risk	10,200
Deutsche Bank Securities, Inc.
Repurchase agreements counterparty risk
Amount at Risk	$ 224,717
Weighted Average Months to Maturity for Repurchase Agreements	1 month
Counterparty risk as percent of Stockholders' Equity, minimum	6.80%