Historic Tax Credits (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended				12 Months Ended		3 Months Ended		12 Months Ended
	Sep. 30, 2013 Phase I Counterparty [Member]	Sep. 30, 2012 Phase I Counterparty [Member]	Sep. 30, 2011 Phase I Counterparty [Member]	Sep. 30, 2010 Phase I Counterparty [Member]	Dec. 31, 2013 Phase I Counterparty [Member]	Sep. 30, 2009 Phase I Counterparty [Member]	Sep. 30, 2013 Phase II Counterparty [Member]	Jun. 30, 2012 Phase II Counterparty [Member]	Dec. 31, 2013 Phase II Counterparty [Member]
Income Taxes [Line Items]
Counterparty contribution of equity						$ 10.6
Amount paid in cash								5.5
Repurchase of counterparty's ownership interest at stated value recorded as Noncontrolling interest					1.6				0.6
Interest and other income	$ 1.8	$ 1.8	$ 1.9	$ 1.7			$ 0.7