Fair Value Measurements (Carrying Values of Assets Measured at Estimated Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value of Financial Instruments
Total available-for-sale securities	$ 447,458	$ 384,359
Total residential mortgage-backed securities
Fair Value of Financial Instruments
Total available-for-sale securities	349,274	287,659
GSE issuance REMICs and CMOs
Fair Value of Financial Instruments
Total available-for-sale securities	333,984	268,998
Non-GSE issuance REMICs and CMOs
Fair Value of Financial Instruments
Total available-for-sale securities	3,556	5,104
GSE pass-through certificates
Fair Value of Financial Instruments
Total available-for-sale securities	11,734	13,557
Obligations of GSEs
Fair Value of Financial Instruments
Total available-for-sale securities	98,182	96,698
Fannie Mae stock
Fair Value of Financial Instruments
Total available-for-sale securities	2	2
Level 1 | Recurring basis
Fair Value of Financial Instruments
Total available-for-sale securities	2	2
Level 1 | Fannie Mae stock | Recurring basis
Fair Value of Financial Instruments
Total available-for-sale securities	2	2
Level 2 | Recurring basis
Fair Value of Financial Instruments
Total available-for-sale securities	447,456	384,357
Level 2 | GSE issuance REMICs and CMOs | Recurring basis
Fair Value of Financial Instruments
Total available-for-sale securities	333,984	268,998
Level 2 | Non-GSE issuance REMICs and CMOs | Recurring basis
Fair Value of Financial Instruments
Total available-for-sale securities	3,556	5,104
Level 2 | GSE pass-through certificates | Recurring basis
Fair Value of Financial Instruments
Total available-for-sale securities	11,734	13,557
Level 2 | Obligations of GSEs | Recurring basis
Fair Value of Financial Instruments
Total available-for-sale securities	$ 98,182	$ 96,698