Financing Receivables (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Financial Receivables [Line Items]
Lease Receivable adjustment		$ 22.0	$ 55.0	$ 20.0	$ 77.0
Provision for Loan and Lease Losses	$ 7.0
Lease investment with non-investment grade counterparties, gross				316.0
Lease investment with non-investment grade counterparties, net of deferred taxes				(83.0)
Expected claim proceeds				31.5
REMA Funded credit support				36.0
Estimated state tax payment related to REMA restructuring				40.0
Estimated federal tax payment related to REMA restructuring				$ 85.0
Powerton Station [Member]
Schedule of Financial Receivables [Line Items]
Counterparties’ S&P Credit Ratings				BB