Commitments and Contingencies (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended		2 Months Ended	3 Months Ended	12 Months Ended
	Apr. 25, 2013	Oct. 31, 2011	Dec. 31, 2011	Mar. 31, 2013	Dec. 31, 2012
Stock Repurchases
Amount authorized under stock repurchase plan		$ 200
Share repurchases made in open market (in shares)	239,712		671,776	539,790	459,252
Average price of shares repurchased (in dollars per share)			$ 48.72	$ 50.17	$ 50.27
Aggregate cost of shares repurchased, excluding broker commissions	11.9		32.7	27.1	23.1
Remaining authorized repurchase amount				$ 117.1