Fair Value of Financial Instruments Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 3,516,239	$ 4,018,161
Derivative assets	5,727	18,488
Derivative Liabilities	35,898	6,681
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	3,516,239	4,018,161
Derivative assets	5,727	18,488
Total assets carried at fair value	3,521,966	4,036,649
Derivative Liabilities	35,898	6,681
Total liabilities carried at fair value	35,898	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	32,896	5,345
Total liabilities carried at fair value	32,896	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	3,472,282	3,944,681
Derivative assets	5,727	18,488
Total assets carried at fair value	3,478,009	3,963,169
Derivative Liabilities	3,002	1,336
Total liabilities carried at fair value	3,002	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	43,957	73,480
Derivative assets	0	0
Total assets carried at fair value	43,957	73,480
Derivative Liabilities	0	0
Total liabilities carried at fair value	$ 0	$ 0