Derivatives and Hedging Transactions (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2011 contract	Dec. 31, 2006 contract	Dec. 31, 2012
Derivatives and Hedging Transactions
Maximum period for hedged transactions	12 months
Number of interest rate swap contracts entered into and subsequently closed		6	2
Fair value of the company's outstanding derivatives
Asset Derivatives	$ 12,100,000			$ 6,500,000
Liability Derivatives	12,700,000			9,900,000
Netting
Fair value of the company's outstanding derivatives
Net liability	600,000			3,400,000
Foreign currency forward contracts
Fair value of the company's outstanding derivatives
Notional values	1,200,000,000			1,300,000,000
Derivatives designated as hedging instruments | Foreign currency forward contracts
Fair value of the company's outstanding derivatives
Asset Derivatives	3,200,000			800,000
Liability Derivatives	1,000,000			1,700,000
Derivatives not designated as hedging instruments | Foreign currency forward contracts
Fair value of the company's outstanding derivatives
Asset Derivatives	8,900,000			5,700,000
Liability Derivatives	$ 11,700,000			$ 8,200,000