Employee Savings Plan (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Employee Savings Plan [Abstract]
Employer contribution expense to 401(k) Savings Plan			$ 4.1	$ 3.2	$ 2.6
Accrued contribution expense to 401(k) Savings Plan			$ 3.6	$ 2.9
Common Stock contributed to the Savings Plan (in shares, thousands)	64	92