Fair Value of Financial Instruments Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 3,949,965	$ 4,018,161
Derivative assets	5,237	18,488
Derivative Liabilities	23,974	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	3,949,965	4,018,161
Derivative assets	5,237	18,488
Total assets carried at fair value	3,955,202	4,036,649
Derivative Liabilities	23,974	6,681
Total liabilities carried at fair value	23,974	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	0	0
Total assets carried at fair value	0	0
Derivative Liabilities	21,139	5,345
Total liabilities carried at fair value	21,139	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	3,884,723	3,944,681
Derivative assets	5,237	18,488
Total assets carried at fair value	3,889,960	3,963,169
Derivative Liabilities	2,835	1,336
Total liabilities carried at fair value	2,835	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	65,242	73,480
Derivative assets	0	0
Total assets carried at fair value	65,242	73,480
Derivative Liabilities	0	0
Total liabilities carried at fair value	$ 0	$ 0