Fair Value Measurements - Valuation of Financial Instruments that are Marked-to-Market on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets
Mortgage-backed securities	$ 190,944	$ 168,934
Derivative instruments	27,493	1,193
Liabilities
Derivative instruments	(29,602)	(24,643)
Fair Value Measurements Recurring
Assets
Mortgage-backed securities	190,944	168,934
Derivative instruments	38,746	5,238
Total earning assets	229,690	174,172
Liabilities
Derivative instruments	(40,855)	(28,688)
Fair Value Measurements Recurring \| Level 1
Assets
Mortgage-backed securities	0	0
Derivative instruments	0	0
Total earning assets	0	0
Liabilities
Derivative instruments	0	0
Fair Value Measurements Recurring \| Level 2
Assets
Mortgage-backed securities	190,944	168,934
Derivative instruments	38,746	5,238
Total earning assets	229,690	174,172
Liabilities
Derivative instruments	(40,855)	(28,688)
Fair Value Measurements Recurring \| Level 3
Assets
Mortgage-backed securities	0	0
Derivative instruments	0	0
Total earning assets	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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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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