Derivative Instruments and Other Hedging Activities - Reconciliation of Gross Amounts in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative assets
Gross derivative assets	$ 11,483	$ 29,056
Gross amounts not offset in the Balance Sheet, derivatives	(548)	(12,469)
Gross amounts not offset in the Balance Sheet, collateral	0	0
Net derivative assets	10,935	16,587
Derivative liabilities
Gross derivative liabilities	31,358	16,191
Gross amounts not offset in the Balance Sheet, derivatives	(548)	(12,469)
Gross amounts not offset in the Balance Sheet, collateral	0	(552)
Net derivative liabilities	30,810	3,170
Written and purchased options
Derivative assets
Gross derivative assets	5,656	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	5,656	8,610
Derivative liabilities
Gross derivative liabilities	5,587
Gross amounts not offset in the Balance Sheet, derivatives	0
Gross amounts not offset in the Balance Sheet, collateral	0
Net derivative liabilities	5,587
Designated as Hedging Instruments | Interest rate contracts
Derivative assets
Gross derivative assets	5,827
Gross amounts not offset in the Balance Sheet, derivatives	(548)
Gross amounts not offset in the Balance Sheet, collateral	0
Net derivative assets	5,279
Derivative liabilities
Gross derivative liabilities	0	0
Gross amounts not offset in the Balance Sheet, derivatives	0	0
Gross amounts not offset in the Balance Sheet, collateral	0	0
Net derivative liabilities	0	0
Not Designated as Hedging Instruments | Interest rate contracts
Derivative assets
Gross derivative assets		20,446
Gross amounts not offset in the Balance Sheet, derivatives		(12,469)
Gross amounts not offset in the Balance Sheet, collateral		0
Net derivative assets		7,977
Derivative liabilities
Gross derivative liabilities	25,771	16,191
Gross amounts not offset in the Balance Sheet, derivatives	(548)	(12,469)
Gross amounts not offset in the Balance Sheet, collateral	0	(552)
Net derivative liabilities	$ 25,223	$ 3,170