Credit Risk and Interest Rate Risk - Offsetting Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Repurchase Agreements
Gross Amounts of Recognized Assets (Liabilities)	$ (7,250,452)	$ (5,600,903)
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	0
Net Amounts Offset in the Consolidated Statements of Financial Position	(7,250,452)	(5,600,903)
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments	8,788,680	7,033,866
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	1,156	2,545
Net Amount	1,539,384	1,435,508
Derivative Asset
Net Amounts Offset in the Consolidated Statements of Financial Position	48,914	9,677
Derivative Liability
Net Amounts Offset in the Consolidated Statements of Financial Position	(320)	(2,350)
Total Liabilities
Gross Amounts of Recognized Assets (Liabilities)	(7,201,858)	(5,594,016)
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	440
Net Amounts Offset in the Consolidated Statements of Financial Position	(7,201,858)	(5,593,576)
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments	8,788,680	7,044,207
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	13,102	45,492
Net Amount	1,599,924	1,496,123
Interest Rate Swaps
Derivative Asset
Gross Amounts of Recognized Assets (Liabilities)	44,874	14,435
Gross Amounts Offset in the Consolidated Statements of Financial Position	(1,617)	(14,435)
Net Amounts Offset in the Consolidated Statements of Financial Position	43,257	0
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	11,570	0
Net Amount	54,827	0
Derivative Liability
Gross Amounts of Recognized Assets (Liabilities)	(1,617)	(14,875)
Gross Amounts Offset in the Consolidated Statements of Financial Position	1,617	14,875
Net Amounts Offset in the Consolidated Statements of Financial Position	0	0
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	0	39,627
Net Amount	0	39,627
Treasury Futures
Derivative Asset
Gross Amounts of Recognized Assets (Liabilities)	3,552	1,785
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	0
Net Amounts Offset in the Consolidated Statements of Financial Position	3,552	1,785
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	376	3,320
Net Amount	3,928	5,105
Derivative Liability
Gross Amounts of Recognized Assets (Liabilities)	0	0
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	0
Net Amounts Offset in the Consolidated Statements of Financial Position	0	0
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	0	0
Net Amount	0	0
Swaptions
Derivative Asset
Gross Amounts of Recognized Assets (Liabilities)	2,105	7,892
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	0
Net Amounts Offset in the Consolidated Statements of Financial Position	2,105	7,892
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	0	0
Net Amount	2,105	7,892
Derivative Liability
Gross Amounts of Recognized Assets (Liabilities)	(320)	(2,350)
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	0
Net Amounts Offset in the Consolidated Statements of Financial Position	(320)	(2,350)
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments	0	10,341
Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	0	0
Net Amount	$ (320)	$ 7,991