Interest rate swap agreements (Details) (Designated as hedging instrument, USD $)	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Sep. 30, 2011	Mar. 31, 2011 Medium term notes issued in March 2011	Mar. 31, 2011 5-year medium-term notes at 4.41%	Mar. 31, 2011 10-year medium-term notes at 5.67%
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)			five-year
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)	5Y
Debt-related disclosures
Medium-term notes issued						$ 125,000,000	$ 75,000,000	$ 50,000,000
Interest rate of medium-term notes (as a percent)							4.41%	5.67%