Derivative Instruments (Narrative) (Details) (USD $)	12 Months Ended	24 Months Ended
	Dec. 31, 2011 years	Apr. 30, 2012	Dec. 31, 2010	Mar. 31, 2010
Hedging gains or losses amortized in accumulated other comprehensive loss	$ 4,800,000
Amortization period of deferred gain (loss) on discontinuation of cash flow hedge	2.25
Outstanding borrowings in interest rate risk	332,000,000		285,000,000
Notional Amount of Interest Rate Derivatives				300,000,000
Borrowings subject to interest rate hedges		250,000,000
Interest rate swaps, interest rate	1.325%
Maximum [Member]
Credit risk of derivatives	4,800,000
Reduction in credit risk of derivatives due to netting feature	4,600,000
One Year from Balance Sheet Date [Member]
Hedging gains or losses amortized in accumulated other comprehensive loss	$ 5,200,000