Balance Sheet Offsetting - Schedule of Balance Sheet Offsetting (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Offsetting Assets Liabilities [Line Items]
Financial liabilities, Gross Amounts Recognized in the Condensed Consolidated Balance Sheets	$ 460,778	$ 609,883
Financial liabilities, Gross Amounts Offset in the Condensed Consolidated Balance Sheets	(890)	(1,072)
Financial liabilities, Net Amounts of Assets presented in the Condensed Consolidated Balance Sheets	459,888	608,811
Financial liabilities, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	890	1,072
Financial liabilities, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Collateral Pledged	(523,129)	(696,213)
Financial liabilities, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Net Amount	(62,351)	(86,330)
Repurchase Agreements [Member]
Offsetting Assets Liabilities [Line Items]
Financial liabilities, Gross Amounts Recognized in the Condensed Consolidated Balance Sheets	455,069	603,028
Financial liabilities, Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Financial liabilities, Net Amounts of Assets presented in the Condensed Consolidated Balance Sheets	455,069	603,028
Financial liabilities, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	0	0
Financial liabilities, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Collateral Pledged	(510,351)	(683,413)
Financial liabilities, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Net Amount	(55,282)	(80,385)
Interest Rate Swaps [Member]
Offsetting Assets Liabilities [Line Items]
Financial assets, Gross Amounts Recognized in the Condensed Consolidated Balance Sheets	4,819	5,783
Financial assets, Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Financial assets, Net Amounts of Assets presented in the Condensed Consolidated Balance Sheets	0	0
Financial assets, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	4,819	5,783
Financial assets, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Collateral Pledged	0	0
Financial assets, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Net Amount	4,819	5,783
Interest Rate Swaps [Member] | Derivatives Not Designated as Hedging Instruments [Member]
Offsetting Assets Liabilities [Line Items]
Financial assets, Gross Amounts Recognized in the Condensed Consolidated Balance Sheets	4,819	5,783
Financial assets, Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Financial assets, Net Amounts of Assets presented in the Condensed Consolidated Balance Sheets	0	0
Financial assets, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	4,819	5,783
Financial assets, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Collateral Pledged	0	0
Financial assets, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Net Amount	4,819	5,783
Financial liabilities, Gross Amounts Recognized in the Condensed Consolidated Balance Sheets	5,709	6,855
Financial liabilities, Gross Amounts Offset in the Condensed Consolidated Balance Sheets	(890)	(1,072)
Financial liabilities, Net Amounts of Assets presented in the Condensed Consolidated Balance Sheets	4,819	5,783
Financial liabilities, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	890	1,072
Financial liabilities, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Collateral Pledged	(12,778)	(12,800)
Financial liabilities, Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Net Amount	$ (7,069)	$ (5,945)