Balance Sheet Offsetting (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Offsetting Liabilities
Gross Amounts Recognized	$ 223,734	$ 206,911
Gross Amounts Offset	0	0
Net Amounts Presented	223,734	206,911
Gross Amounts Not Offset, Financial Instruments	(3,509)	(5,268)
Gross Amounts Not Offset, Cash Collateral	(210,187)	(201,643)
Net Amount	10,038	0
Collateral posted with derivatives counterparties	57,700	52,400
Repurchase agreements
Offsetting Repurchase Agreement Liabilities
Gross Amounts Recognized	200,000	165,000
Gross Amounts Offset	0	0
Net Amounts Presented	200,000	165,000
Gross Amounts Not Offset, Financial Instruments	0	0
Gross Amounts Not Offset, Cash Collateral	(200,000)	(165,000)
Net Amount	0	0
Interest rate caps and swaps
Offsetting Assets
Gross Amounts Recognized	26,513	26,087
Gross Amounts Offset	0	0
Net Amounts Presented	26,513	26,087
Gross Amounts Not Offset, Financial Instruments	(3,509)	(5,268)
Gross Amounts Not Offset, Cash Collateral	0	0
Net Amount	23,004	20,819
Offsetting Derivative Liabilities
Gross Amounts Recognized	23,734	41,911
Gross Amounts Offset	0	0
Net Amounts Presented	23,734	41,911
Gross Amounts Not Offset, Financial Instruments	(3,509)	(5,268)
Gross Amounts Not Offset, Cash Collateral	(10,187)	(36,643)
Net Amount	$ 10,038	$ 0