Derivatives and Hedging Activities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Decrease (increase) to interest expense	$ 6,500	$ 6,300
Amount reclassified to interest expenses within Next 12 months	25,200
Unrealized gain on discontinued cash flow hedges included in AOCI	2,497,668		$ 2,630,491
Notional amount	10,110,532		9,250,352
Derivative Liabilities	8,204		22,445
Cash margin deposits	5,800		620
Agency RMBS
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Cash margin deposits	8,400
Derivative and hedging attributable to Parent
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Unrealized gain on discontinued cash flow hedges included in AOCI	117,400		123,900
Currency Forward Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount	77,836		76,859
Derivative Liabilities	184		1,217
Pound Sterling | Currency Forward Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount	51,400		49,700
Euro | Currency Forward Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount	$ 26,500		$ 27,200