Fair Value of Financial Instruments Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 3,328,752	$ 3,493,701
Derivative assets	14,845	7,835
Derivative liabilities	90,487	41,205
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	3,328,752	3,493,701
Derivative assets	14,845	7,835
Total assets carried at fair value	3,343,597	3,501,536
Derivative liabilities	90,487	41,205
Total liabilities carried at fair value	90,487	41,205
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	41,283	29,097
Total liabilities carried at fair value	41,283	29,097
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	3,314,046	3,477,266
Derivative assets	14,845	7,835
Total assets carried at fair value	3,328,891	3,485,101
Derivative liabilities	49,204	12,108
Total liabilities carried at fair value	49,204	12,108
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	14,706	16,435
Derivative assets	0	0
Total assets carried at fair value	14,706	16,435
Derivative liabilities	0	0
Total liabilities carried at fair value	$ 0	$ 0