Stock Repurchases (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Feb. 13, 2017
Equity, Class of Treasury Stock [Line Items]
Increase in number of shares authorized for repurchase						1,000,000
Remaining number of shares authorized for repurchase		776,208		776,208
Common stock repurchased, shares				6,185	610,260
Common stock repurchased, value				$ 2.0	$ 123.5
Open Market [Member]
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased, shares	[1]	0	81,737	0	588,580
Common stock repurchased, value	[1]	$ 0.0	$ 17.7	$ 0.0	$ 119.1
Other [Member]
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased, shares	[2]			6,185	21,680
Common stock repurchased, value	[2]			$ 2.0	$ 4.4

[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 June 30, 2018, 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 and the conversion of restricted stock units to common stock.