Offsetting Assets and Liabilities - Offsetting of Derivative Liabilities (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Derivatives:
Gross amounts of recognized liabilities, derivatives	$ 134,228,000	$ 238,148,000
Gross amounts offset in the consolidated balance sheet, derivatives	0	0
Net amounts of liabilities presented in the consolidated balance sheet, derivatives	134,228,000	238,148,000
Financial instruments, derivatives	(45,738,000)	(117,240,000)
Collateral received, derivatives	(85,787,000)	(109,299,000)
Net amount, derivatives	2,703,000	11,609,000
Repurchase Agreements:
Gross amounts of recognized liabilities, repurchase agreements	11,160,669,000	12,126,048,000
Gross amounts offset in the consolidated balance sheet, repurchase agreements	0	0
Net amounts of assets presented in the consolidated balance sheet, repurchase agreements	11,160,669,000	12,126,048,000
Financial instruments, repurchase agreements	(11,160,669,000)	(12,126,048,000)
Collateral received, repurchase agreements	0	0
Net amount, repurchase agreements	0	0
Secured Loans:
Gross amounts of recognized liabilities, secured loans	1,650,000,000	1,650,000,000
Gross amounts offset in the consolidated balance sheet, secured loans	0	0
Net amounts of assets presented in the consolidated balance sheet, secured loans	1,650,000,000	1,650,000,000
Financial instruments, secured loans	(1,650,000,000)	(1,650,000,000)
Collateral received, secured loans	0	0
Net amount, secured loans	0	0
Total gross amount of recognized liabilities	12,944,897,000	14,014,196,000
Total gross amounts offset in the consolidated balance sheets	0	0
Net amounts of assets presented in the consolidated balance sheets	12,944,897,000	14,014,196,000
Total financial instruments	(12,856,407,000)	(13,893,288,000)
Total collateral received	(85,787,000)	(109,299,000)
Total net amount	2,703,000	11,609,000
Securities sold under agreements to repurchase fair value of collateral	12,444,304,000	13,460,998,000
Cash collateral received	1,700,000	4,900,000
Non-cash collateral received	536,000	0
Due from counterparties	86,450,000	110,009,000
Collateral pledged against secured loans	1,900,000,000	1,900,000,000
Interest Rate Swaps
Derivatives:
Gross amounts of recognized liabilities, derivatives	133,833,000	238,045,000
Secured Loans:
Cash collateral received	1,700,000	4,900,000
Non-cash collateral received	536,000	0
Due from counterparties	$ 86,500,000	$ 109,300,000