Fair Value of Financial Instruments Fair Value on a Recurring Basis (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, at fair value	$ 4,559,323	$ 4,103,981
Derivative assets	14,860	0
Derivative Liabilities	21,192	42,537
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities, at fair value	4,559,323	4,103,981
Other Investments, Fair Value Disclosure	25	25
Derivative assets	14,860
Total assets carried at fair value	4,574,208	4,104,006
Derivative Liabilities	21,192	42,537
Total liabilities carried at fair value	21,192	42,537
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities, at fair value	0	0
Other Investments, Fair Value Disclosure	0	0
Derivative assets	0
Total assets carried at fair value	0	0
Derivative Liabilities	0	0
Total liabilities carried at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities, at fair value	4,446,551	3,998,761
Other Investments, Fair Value Disclosure	0	0
Derivative assets	14,860
Total assets carried at fair value	4,461,411	3,998,761
Derivative Liabilities	21,192	42,537
Total liabilities carried at fair value	21,192	42,537
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities, at fair value	112,772	105,220
Other Investments, Fair Value Disclosure	25	25
Derivative assets	0
Total assets carried at fair value	112,797	105,245
Derivative Liabilities	0	0
Total liabilities carried at fair value	$ 0	$ 0