Interest rate swap agreements (Details) (Designated as hedging instrument, USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Mar. 24, 2011	Jun. 30, 2010 Medium term notes issued in March 2011
Interest rate swap agreements
Notional amounts of interest rate derivatives				$ 125,000,000
Reference rate of interest rate derivative	three-month LIBOR
Period over which entity hedges its interest rate derivative (in years)	5 years
Fair value of interest rate derivative recorded in other liabilities		(2,800,000)
Payment on settlement of interest rate derivatives	5,200,000
Gain (loss) due to ineffective portion of interest rate, cumulative effect, recognized in earnings	(3,300,000)
Gain (loss) due to ineffective portion of interest rate, recognized in earnings	(2,500,000)	(800,000)
Amount amortized to interest expenses upon settlement	(1,900,000)
Amortization period of forward starting swaps (in years)	5 years
Debt-related disclosures
Medium-term notes issued			$ 125,000,000