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Share Repurchase Program	9 Months Ended
Share Repurchase Program	Sep. 30, 2017
Equity [Abstract]
Share Repurchase Program	Share Repurchase Program We have an ongoing authorization, originally approved by our board of directors in September 2010 and subsequently amended, to repurchase up to $1.0 billion in shares of our outstanding common stock. The authorization expires on December 31, 2017. We may repurchase shares from time to time at prevailing market prices on the open market or in private transactions in amounts that we deem appropriate. As of September 30, 2017, we had repurchased a total of 10,561,496 shares for $713.5 million under this authorization, leaving approximately $286.5 million available for future repurchases.