Fair Value Measurements - Valuation of Financial Instruments that are Marked-to-Market on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
Mortgage-backed securities	$ 195,391	$ 168,934
Derivative instruments	6,036	1,193
Liabilities
Derivative instruments	(21,219)	(24,643)
Fair Value Measurements Recurring
Assets
Mortgage-backed securities	195,391	168,934
Derivative instruments	15,314	5,238
Total	210,705	174,172
Liabilities
Derivative instruments	(30,497)	(28,688)
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	195,391	168,934
Derivative instruments	15,314	5,238
Total	210,705	174,172
Liabilities
Derivative instruments	(30,497)	(28,688)
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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