Historic Tax Credits (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended			12 Months Ended			37 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2015	Dec. 31, 2013	Dec. 31, 2009	Sep. 30, 2013
Income Taxes [Line Items]
Counterparty contribution of equity	$ 3.2
Payment to repurchase ownership interest			$ 1.8
Phase I Counterparty [Member]
Income Taxes [Line Items]
Counterparty contribution of equity								$ 10.6
Tax Credit Amortization Income				$ 1.8	$ 1.8
Priority Return Expense				1.2					$ 1.0
Phase II Counterparty [Member]
Income Taxes [Line Items]
Counterparty contribution of equity				5.8
Amount paid in cash		$ 5.5				$ 1.7
Repurchase of counterparty's ownership interest at stated value recorded as Noncontrolling interest						1.6
Priority Return Expense						0.3			$ 0.1
Interest and Other Income			1.8	1.9	$ 0.7
Phase II(i) Counterparty [Member]
Income Taxes [Line Items]
Tax Credit Amortization Income			$ 0.9	1.2			$ 0.7
Phase II(ii) Counterparty [Member]
Income Taxes [Line Items]
Interest and other income				$ 1.2		$ 1.0