Derivative Financial Instruments (Counterparty Credit Risk Narrative) (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Derivative [Line Items]
Derivative, Collateral, Right to Reclaim Cash	$ 20.7
Derivative instrument asset, Amount Offset	30.5
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	(9.8)
Credit Derivative, Maximum Exposure, Undiscounted	35.1
Market Approach Valuation Technique [Member]
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	$ 27.7