Fair Value of Financial Instruments Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 3,959,852	$ 4,018,161
Derivative assets	12,064	18,488
Derivative Liabilities	11,137	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	3,959,852	4,018,161
Other Investments, Fair Value Disclosure		18,488
Derivative assets	12,064
Total assets carried at fair value	3,971,916	4,036,649
Derivative Liabilities	11,137	6,681
Total liabilities carried at fair value	11,137	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
Other Investments, Fair Value Disclosure		0
Derivative assets	0
Total assets carried at fair value	0	0
Derivative Liabilities	9,759	5,345
Total liabilities carried at fair value	9,759	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	3,893,465	3,944,681
Other Investments, Fair Value Disclosure		18,488
Derivative assets	12,064
Total assets carried at fair value	3,905,529	3,963,169
Derivative Liabilities	1,378	1,336
Total liabilities carried at fair value	1,378	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	66,387	73,480
Other Investments, Fair Value Disclosure		0
Derivative assets	0
Total assets carried at fair value	66,387	73,480
Derivative Liabilities	0	0
Total liabilities carried at fair value	$ 0	$ 0