Minimum Lease Payment Obligations under Operating Leases with Terms in Excess of One Year (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Operating Leases [Line Items]
Lease expense, straight-line recognition period	44 years	44 years	44 years
Lease expense, recognized	$ 7.1	$ 7.1	$ 7.4
Increase in deferred lease liability	4.1	4.2
Estimated deferred lease liability through 2032	64.7
Remaining lease term after 2032	19 years
Natural Gas Storage
Schedule of Operating Leases [Line Items]
Deferred lease liability	$ 17.5	$ 13.3