Fair Value of Financial Instruments Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 3,186,749	$ 3,212,084
Derivative assets	272	28,534
Total assets carried at fair value	3,187,021	3,240,618
Derivative liabilities	58	6,922
Total liabilities carried at fair value	58	6,922
Level 1
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	0	0
Total liabilities carried at fair value	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	3,176,247	3,201,157
Derivative assets	272	28,534
Total assets carried at fair value	3,176,519	3,229,691
Derivative liabilities	58	6,922
Total liabilities carried at fair value	58	6,922
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	10,502	10,927
Derivative assets	0	0
Total assets carried at fair value	10,502	10,927
Derivative liabilities	0	0
Total liabilities carried at fair value	$ 0	$ 0