Equity Offering (Details) (Equity Program [Member], USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Mar. 31, 2012	Sep. 14, 2011
Equity Program [Member]
Equity Offering Program [Line Items]
Common stock value authorized under equity offering program			$ 125
Common stock shares issued under equity offering program	1,166,875
Weighted average issue price of common stock	$ 40.59
Proceeds from issuance of common stock	46.4
Offering related costs and commissions	0.9
Expenses related to Equity Program	0.4
Common stock, value reserved for future issuance		$ 77.6