Related party transactions with affiliated company - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended	9 Months Ended		12 Months Ended						9 Months Ended
	Sep. 30, 2012 Prlp2011 Holdings Llc Joint Venture [Member]	Sep. 30, 2011 Prlp2011 Holdings Llc Joint Venture [Member]	Dec. 31, 2011 Prlp2011 Holdings Llc Joint Venture [Member]	Sep. 30, 2012 E V E R T E C Inc [Member]	Sep. 30, 2010 E V E R T E C Inc [Member]	Dec. 31, 2011 E V E R T E C Inc [Member]	Sep. 30, 2011 E V E R T E C Inc [Member]	Sep. 30, 2012 Performance Guarantee [Member] E V E R T E C Inc [Member]	Dec. 31, 2011 Performance Guarantee [Member] E V E R T E C Inc [Member]	Sep. 30, 2012 Financial Standby Letter Of Credit Member	Dec. 31, 2011 Financial Standby Letter Of Credit Member	Sep. 30, 2012 Financial Standby Letter Of Credit Member E V E R T E C Inc [Member]	Dec. 31, 2011 Financial Standby Letter Of Credit Member E V E R T E C Inc [Member]
Related Party Transaction [Line Items]
Sale of majority interest in subsidiaries, date of sale					Sep. 30, 2010
Percentage Of Ownership Interests Sold				51.50%	51.00%	51.00%	51.00%
Equity Method Investment Ownership Percentage	24.90%	24.90%	24.90%	48.50%	49.00%	49.00%	49.00%
Guarantee Obligations Current Carrying Value								$ 7.7	$ 15.0	$ 0.7	$ 0.5	$ 2.9	$ 2.9
Guarantee Obligations Term												5-year period
Majority Interest Percentage	75.10%	75.10%	75.10%
Equity method investments, distributions				$ 131		$ 0