Fair Value of Financial Instruments Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 3,852,883	$ 3,516,239
Derivative assets	20,804	5,727
Derivative liabilities	48,240	35,898
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	3,852,883	3,516,239
Derivative assets	20,804	5,727
Total assets carried at fair value	3,873,687	3,521,966
Derivative liabilities	48,240	35,898
Total liabilities carried at fair value	48,240	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	43,991	32,896
Total liabilities carried at fair value	43,991	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	3,813,929	3,472,282
Derivative assets	20,804	5,727
Total assets carried at fair value	3,834,733	3,478,009
Derivative liabilities	4,249	3,002
Total liabilities carried at fair value	4,249	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	38,954	43,957
Derivative assets	0	0
Total assets carried at fair value	38,954	43,957
Derivative liabilities	0	0
Total liabilities carried at fair value	$ 0	$ 0