Fair Value Measurements - Valuation of Financial Instruments that are Marked-to-Market on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets
Mortgage-backed securities	$ 208,603	$ 195,391
Derivative instruments	0	6,036
Fair Value Measurements Recurring
Assets
Mortgage-backed securities	208,603	195,391
Derivative instruments	7,808	15,314
Total	216,411	210,705
Liabilities
Derivative instruments	(25,937)	(30,497)
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	208,603	195,391
Derivative instruments	7,808	15,314
Total	216,411	210,705
Liabilities
Derivative instruments	(25,937)	(30,497)
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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