Stockholders' Equity (Details 2) (USD $) In Millions, except Share data, unless otherwise specified			0 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Jan. 15, 2013 Subsequent event
Common Stock Offering
Common stock number of shares sold			4,400,000
Sale of stock, price per share (in dollars per share)			$ 80.09
Proceeds from issuance of common stock			$ 352.4
Preferred Stock
Preferred stock, shares authorized	5,000,000	5,000,000
Number of series of preferred stock authorized, minimum	1
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001