Derivative Instruments and Other Hedging Activities - Reconciliation of Gross Amounts in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative assets
Gross derivative assets	$ 9,656	$ 29,056
Gross amounts not offset in the Balance Sheet, derivatives	(310)	(12,469)
Gross amounts not offset in the Balance Sheet, collateral	0	0
Net derivative assets	9,346	16,587
Derivative liabilities
Gross derivative liabilities	37,781	16,191
Gross amounts not offset in the Balance Sheet, derivatives	(310)	(12,469)
Gross amounts not offset in the Balance Sheet, collateral	0	(552)
Net derivative liabilities	37,471	3,170
Written and purchased options
Derivative assets
Gross derivative assets	5,268	8,610
Gross amounts not offset in the Balance Sheet, derivatives	0	0
Gross amounts not offset in the Balance Sheet, collateral	0	0
Net derivative assets	5,268	8,610
Derivative liabilities
Gross derivative liabilities	5,121
Gross amounts not offset in the Balance Sheet, derivatives	0
Gross amounts not offset in the Balance Sheet, collateral	0
Net derivative liabilities	5,121
Designated as Hedging Instruments | Interest rate contracts
Derivative assets
Gross derivative assets	305
Gross amounts not offset in the Balance Sheet, derivatives	0
Gross amounts not offset in the Balance Sheet, collateral	0
Net derivative assets	305
Derivative liabilities
Gross derivative liabilities		0
Gross amounts not offset in the Balance Sheet, derivatives		0
Gross amounts not offset in the Balance Sheet, collateral		0
Net derivative liabilities		0
Not Designated as Hedging Instruments | Interest rate contracts
Derivative assets
Gross derivative assets	4,083	20,446
Gross amounts not offset in the Balance Sheet, derivatives	(310)	(12,469)
Gross amounts not offset in the Balance Sheet, collateral	0	0
Net derivative assets	3,773	7,977
Derivative liabilities
Gross derivative liabilities	32,660	16,191
Gross amounts not offset in the Balance Sheet, derivatives	(310)	(12,469)
Gross amounts not offset in the Balance Sheet, collateral	0	(552)
Net derivative liabilities	$ 32,350	$ 3,170