Stock Repurchases (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015		Feb. 13, 2017
Equity, Class of Treasury Stock [Line Items]
Increase in number of shares authorized for repurchase								1,000,000
Number of shares authorized for repurchase		776,208
Common stock repurchased, shares		610,260		666,330		470,071
Common stock repurchased, value		$ 123.5		$ 121.7		$ 86.5
Open Market [Member]
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased, shares	[1]	588,580		495,662		464,006
Common stock repurchased, value	[1]	$ 119.1		$ 88.5		$ 85.4
Other [Member]
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased, shares	[2]	21,680		170,668		6,065
Common stock repurchased, value		$ 4.4	[2]	$ 33.2	[2]	$ 1.1	[3]

[1] Represents repurchases under authorizations by the board of directors for the repurchase of shares by us from time to time in the open market or in privately negotiated transactions. On February 13, 2017, the board of directors authorized the repurchase of up to one million shares of our common stock in addition to the board’s prior authorizations. As of December 31, 2017, we had authorization to repurchase 776,208 shares of our common stock.

[2] Represents shares of common stock released to us by team members as payment of tax withholdings upon the vesting of restricted stock and restricted stock units.

[3] Represents shares of common stock released to us by team members as payment of tax withholdings upon the vesting of restricted stock and restricted stock units.