Fair Value of Financial Instruments Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 3,737,729	$ 3,516,239
Derivative assets	10,057	5,727
Derivative liabilities	63,708	35,898
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	3,737,729	3,516,239
Derivative assets	10,057	5,727
Total assets carried at fair value	3,747,786	3,521,966
Derivative liabilities	63,708	35,898
Total liabilities carried at fair value	63,708	35,898
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	33,076	32,896
Total liabilities carried at fair value	33,076	32,896
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	3,715,096	3,472,282
Derivative assets	10,057	5,727
Total assets carried at fair value	3,725,153	3,478,009
Derivative liabilities	30,632	3,002
Total liabilities carried at fair value	30,632	3,002
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	22,633	43,957
Derivative assets	0	0
Total assets carried at fair value	22,633	43,957
Derivative liabilities	0	0
Total liabilities carried at fair value	$ 0	$ 0