Fair Value of Financial Instruments Recorded basis and Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities, Fair Value Disclosure	$ 4,530,289	$ 4,073,912
Derivative assets	14,860	0
Derivative Liabilities, Fair Value Disclosure	21,192	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	4,559,323	4,103,981
Securitized mortgage loans, Fair Value Disclosure	62,083	70,823
Other Investments, Fair Value Disclosure	5,782	858
Derivative assets	14,860	0
Short-term Debt, Fair Value	4,071,392	3,564,128
Debt Instrument, Fair Value Disclosure	24,634	30,504
Derivative Liabilities, Fair Value Disclosure	21,192	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	4,559,323	4,103,981
Securitized mortgage loans, Fair Value Disclosure	52,313	61,916
Other Investments, Fair Value Disclosure	5,782	810
Derivative assets	14,860	0
Short-term Debt, Fair Value	4,071,843	3,564,787
Debt Instrument, Fair Value Disclosure	24,539	30,756
Derivative Liabilities, Fair Value Disclosure	$ 21,192	$ 42,537