Fair Value of Financial Instruments Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 3,615,393	$ 4,018,161
Derivative assets	7,297	18,488
Derivative Liabilities	18,058	6,681
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	3,615,393	4,018,161
Derivative assets	7,297	18,488
Total assets carried at fair value	3,622,690	4,036,649
Derivative Liabilities	18,058	6,681
Total liabilities carried at fair value	18,058	6,681
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	0	0
Derivative assets	339	0
Total assets carried at fair value	339	0
Derivative Liabilities	17,621	5,345
Total liabilities carried at fair value	17,621	5,345
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	3,563,819	3,944,681
Derivative assets	6,958	18,488
Total assets carried at fair value	3,570,777	3,963,169
Derivative Liabilities	437	1,336
Total liabilities carried at fair value	437	1,336
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	51,574	73,480
Derivative assets	0	0
Total assets carried at fair value	51,574	73,480
Derivative Liabilities	0	0
Total liabilities carried at fair value	$ 0	$ 0