Derivative Financial Instruments (Offsetting Derivatives) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative instrument assets, Gross Amount	$ 6,424	$ 35,727
Derivative instrument asset, Amount Offset	30,500
Derivative Asset	969	0
Liability Derivatives, Fair Value, Gross Liability	3,838	25,300
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	(9,800)
Derivative liability	156	446
Hedge Accounting Positions [Member]
Derivative [Line Items]
Derivative instrument assets, Gross Amount	1,972	3,270
Derivative instrument asset, Amount Offset, Total	(292)	(2,335)
Derivative instrument asset, Amount Offset	1,051	935
Derivative Asset	629	0
Liability Derivatives, Fair Value, Gross Liability	195	792
Derivative instrument liability, Amount Offset, Total	(195)	(792)
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0	0
Derivative liability	0	0
Non-Hedged Accounting Positions [Member]
Derivative [Line Items]
Derivative instrument assets, Gross Amount	4,452	32,457
Derivative instrument asset, Amount Offset, Total	(2,623)	(21,917)
Derivative instrument asset, Amount Offset	1,489	10,540
Derivative Asset	340	0
Liability Derivatives, Fair Value, Gross Liability	3,643	24,508
Derivative instrument liability, Amount Offset, Total	(2,721)	(23,462)
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	(766)	(600)
Derivative liability	156	446
Subject to and not subject to Master Netting Agreements [Member]
Derivative [Line Items]
Derivative instrument asset, Amount Offset, Total	(2,915)	(24,252)
Derivative instrument asset, Amount Offset	2,540	11,475
Derivative Asset	36,795	41,910
Derivative instrument liability, Amount Offset, Total	(2,916)	(24,254)
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	(766)	(600)
Derivative liability	$ 16,341	$ 20,335