Fair Value of Financial Instruments Recorded basis and Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities, Fair Value Disclosure	$ 3,989,444	$ 4,073,912
Derivative assets	18,488	0
Derivative Liabilities, Fair Value Disclosure	6,681	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	4,018,161	4,103,981
Securitized mortgage loans, Fair Value Disclosure	54,748	70,823
Other Investments, Fair Value Disclosure	675	858
Derivative assets	18,488	0
Short-term Debt, Fair Value	3,580,754	3,564,128
Debt Instrument, Fair Value Disclosure	12,914	30,504
Derivative Liabilities, Fair Value Disclosure	6,681	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	4,018,161	4,103,981
Securitized mortgage loans, Fair Value Disclosure	45,750	61,916
Other Investments, Fair Value Disclosure	633	810
Derivative assets	18,488	0
Short-term Debt, Fair Value	3,580,997	3,564,787
Debt Instrument, Fair Value Disclosure	12,414	30,756
Derivative Liabilities, Fair Value Disclosure	$ 6,681	$ 42,537