Offsetting Assets and Liabilities - Offsetting of Derivative Liabilities (Detail) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Derivatives Liability
Gross Amounts of Recognized Liabilities	$ 44,147,000	$ 134,228,000
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts of Liabilities presented in the Condensed Consolidated Balance Sheets	44,147,000	134,228,000
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	(43,047,000)	(45,738,000)
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Collateral Posted	(980,000)	(85,787,000)
Net Amount	120,000	2,703,000
Repurchase Agreements
Gross Amounts of Recognized Liabilities	12,118,948,000	11,160,669,000
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts of Liabilities presented in the Condensed Consolidated Balance Sheets	12,118,948,000	11,160,669,000
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	(12,118,948,000)	(11,160,669,000)
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Collateral Posted	0	0
Net Amount	0	0
Secured Loans
Gross Amounts of Recognized Liabilities	1,650,000,000	1,650,000,000
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts of Liabilities presented in the Condensed Consolidated Balance Sheets	1,650,000,000	1,650,000,000
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	(1,650,000,000)	(1,650,000,000)
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Collateral Posted	0	0
Net Amount	0	0
Total Borrowings
Gross Amounts of Recognized Liabilities	13,813,095,000	12,944,897,000
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts of Liabilities presented in the Condensed Consolidated Balance Sheets	13,813,095,000	12,944,897,000
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	(13,811,995,000)	(12,856,407,000)
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Collateral Posted	(980,000)	(85,787,000)
Net Amount	120,000	2,703,000
Fair value of securities pledged under repurchase agreement	13,370,297,000	12,444,304,000
Cash collateral posted by Company's derivative counterparties	3,471,000	1,700,000
Non-cash collateral received on derivatives	0	536,000
Due from counterparties		86,450,000
Collateral pledged against secured loans	$ 1,941,665,000	$ 1,931,582,000