Organization and Significant Accounting Policies - Gross Unrealized Losses and Fair Value of individual securities in Mortgage Backed Securities portfolio (Detail) $ in Thousands	Jun. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item	103	120
Less Than 12 Months Fair Value	$ 1,140,396	$ 978,041
Less Than 12 Months Unrealized Losses	$ (9,836)	$ (12,961)
12 Months or More Number of Securities | item	149	261
12 Months or More Fair Value	$ 376,478	$ 472,090
12 Months or More Unrealized Losses	$ (5,907)	$ (8,556)
Total Number of Securities | item	252	381
Total Fair Value	$ 1,516,874	$ 1,450,131
Total Unrealized Losses	$ (15,743)	$ (21,517)
Non-Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item	5	30
Less Than 12 Months Fair Value	$ 14,783	$ 163,741
Less Than 12 Months Unrealized Losses	$ (149)	$ (5,911)
12 Months or More Number of Securities | item	29	36
12 Months or More Fair Value	$ 134,151	$ 206,516
12 Months or More Unrealized Losses	$ (5,320)	$ (4,551)
Total Number of Securities | item	34	66
Total Fair Value	$ 148,934	$ 370,257
Total Unrealized Losses	$ (5,469)	$ (10,462)