Liabilities and Assets Subject to Netting Arrangements (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative Assets Derivative Liabilities And Repurchase Agreements Subject To Netting Agreements [Line Items]
Pledged in Agency MBS as collateral		$ 30,300
Paid swap margin calls on our derivatives	$ 75,500	40,200
Derivative counterparty margin	5,238
Derivative instruments at fair value	27,396	46,207
Derivative instruments at fair value	36,261	15,901
Agency MBS
Derivative Assets Derivative Liabilities And Repurchase Agreements Subject To Netting Agreements [Line Items]
Pledged in Agency MBS as collateral		3,430,000
Non-Agency MBS
Derivative Assets Derivative Liabilities And Repurchase Agreements Subject To Netting Agreements [Line Items]
Pledged in Agency MBS as collateral		726,400
Residential mortgage loans
Derivative Assets Derivative Liabilities And Repurchase Agreements Subject To Netting Agreements [Line Items]
Pledged in Agency MBS as collateral	17,200
Interest rate swap agreements
Derivative Assets Derivative Liabilities And Repurchase Agreements Subject To Netting Agreements [Line Items]
Derivative instruments at fair value	$ 23,000
Interest rate swap agreements | Collateral Pledged
Derivative Assets Derivative Liabilities And Repurchase Agreements Subject To Netting Agreements [Line Items]
Derivative instruments at fair value		$ 15,900