Fair Value of Financial Instruments Fair Value on a Recurring Basis (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, at fair value	$ 4,141,744	$ 4,103,981
Derivative assets	12,908	0
Derivative Liabilities	20,837	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	4,141,744	4,103,981
Other Investments, Fair Value Disclosure		25
Derivative assets	12,908
Total assets carried at fair value	4,154,652	4,104,006
Derivative Liabilities	20,837	42,537
Total liabilities carried at fair value	20,837	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
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	4,039,331	3,998,761
Other Investments, Fair Value Disclosure		0
Derivative assets	12,908
Total assets carried at fair value	4,052,239	3,998,761
Derivative Liabilities	20,837	42,537
Total liabilities carried at fair value	20,837	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	102,413	105,220
Other Investments, Fair Value Disclosure		25
Derivative assets	0
Total assets carried at fair value	102,413	105,245
Derivative Liabilities	0	0
Total liabilities carried at fair value	$ 0	$ 0