Stockholder's Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified					12 Months Ended	0 Months Ended			3 Months Ended	0 Months Ended			12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 2% Senior Convertible Notes Due 2017	Mar. 26, 2007 2% Senior Convertible Notes Due 2017	Dec. 31, 2011 Common Stock	Sep. 07, 2012 Common Stock and-or Convertible Debt	Sep. 14, 2011 Common Stock and-or Convertible Debt	Mar. 09, 2011 Common Stock and-or Convertible Debt	Mar. 31, 2013 Common Stock and-or Convertible Debt	Sep. 07, 2012 Common Stock and-or Convertible Debt 2% Senior Convertible Notes Due 2017	Sep. 14, 2011 Common Stock and-or Convertible Debt 2% Senior Convertible Notes Due 2017	Mar. 09, 2011 Common Stock and-or Convertible Debt 2% Senior Convertible Notes Due 2017	Dec. 31, 2012 Open Market Purchases And Pre-Established Trading Plan Common Stock
Equity, Class of Treasury Stock
Preferred stock, shares authorized	5,000,000	5,000,000
Share repurchase program, authorized amount						$ 200	$ 200	$ 150
Share repurchase program, remaining authorized repurchase amount									200.0
Interest rate on senior convertible notes (as a percent)			2.00%	2.00%						2.00%	2.00%	2.00%
Convertible notes maturity date										2017	2017	2017
Total number of shares repurchased					9,200,000								6,900,000
Average price per share of common stock repurchased (in dollars per shares)					$ 18.52								$ 26.20
Cost of reacquired shares					$ 169.7								$ 180.3