Fair Value of Financial Instruments Recorded basis and Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities, Fair Value Disclosure	$ 4,112,766	$ 4,073,912
Derivative assets	12,908	0
Derivative Liabilities, Fair Value Disclosure	20,837	42,537
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities, Fair Value Disclosure	4,141,744	4,103,981
Securitized mortgage loans, Fair Value Disclosure	59,797	70,823
Other Investments, Fair Value Disclosure	1,305	858
Derivative assets	12,908	0
Short-term Debt, Fair Value	3,674,850	3,564,128
Debt Instrument, Fair Value Disclosure	21,148	30,504
Derivative Liabilities, Fair Value Disclosure	20,837	42,537
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities, Fair Value Disclosure	4,141,744	4,103,981
Securitized mortgage loans, Fair Value Disclosure	50,932	61,916
Other Investments, Fair Value Disclosure	1,305	810
Derivative assets	12,908	0
Short-term Debt, Fair Value	3,675,197	3,564,787
Debt Instrument, Fair Value Disclosure	20,855	30,756
Derivative Liabilities, Fair Value Disclosure	$ 20,837	$ 42,537