Repurchase Agreements - Outstanding, Weighted Average Borrowing Rates, Weighted Average Remaining Maturities, Average Daily Balances and the Fair Value of Collateral Pledged (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	$ 9,127,048	$ 7,250,452
Repurchase Agreements | Agency MBS - Residential
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	5,569,418	3,202,056
Average balance of Repurchase agreements secured by:	$ 3,907,750	$ 3,120,474
Average borrowing rate of Repurchase agreements secured by:	2.09%	1.47%
Average remaining maturity of Repurchase agreements secured by:	24 days	28 days
Average original maturity of Repurchase agreements secured by:	66 days	87 days
MBS pledged as collateral at fair value on Repurchase agreements:	$ 5,954,878	$ 3,450,123
Repurchase Agreements | Non-agency MBS and Loans held for investment (in thousands)
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	3,557,630	4,048,396
Average balance of Repurchase agreements secured by:	$ 3,893,703	$ 3,203,700
Average borrowing rate of Repurchase agreements secured by:	4.02%	3.41%
Average remaining maturity of Repurchase agreements secured by:	98 days	93 days
Average original maturity of Repurchase agreements secured by:	124 days	123 days
MBS pledged as collateral at fair value on Repurchase agreements:	$ 4,671,505	$ 5,338,557
Repurchase Agreements | Residential Mortgage-Backed Securities
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	9,127,048	7,250,452
Average balance of Repurchase agreements secured by:	7,801,453	6,324,174
MBS pledged as collateral at fair value on Repurchase agreements:	$ 10,626,383	$ 8,788,680