Fair Value Measurements - Valuation of Financial Instruments that are Marked-to-Market on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Mortgage-backed securities	$ 244,088	$ 208,603
Derivative instruments	0	0
Fair Value Measurements Recurring
Assets
Mortgage-backed securities	244,088	208,603
Derivative instruments	812	7,808
Total	244,900	216,411
Liabilities
Derivative instruments	(2,584)	(25,937)
Fair Value Measurements Recurring \| Level 1
Assets
Mortgage-backed securities	0	0
Derivative instruments	0	0
Total	0	0
Liabilities
Derivative instruments	0	0
Fair Value Measurements Recurring \| Level 2
Assets
Mortgage-backed securities	244,088	208,603
Derivative instruments	812	7,808
Total	244,900	216,411
Liabilities
Derivative instruments	(2,584)	(25,937)
Fair Value Measurements Recurring \| Level 3
Assets
Mortgage-backed securities	0	0
Derivative instruments	0	0
Total	0	0
Liabilities
Derivative instruments	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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