Stockholders' Equity - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Numerator used in basic and diluted EPS:
Net income	$ 153.5	$ 121.0	$ 286.5	$ 229.8	$ 555.1
Denominator:
Weighted average number of common shares used in basic EPS (in shares)	165,015,642	164,922,237	165,029,345	165,682,614
Effect of dilutive shares:
Potential common shares issuable from stock options and stock awards (in shares)	3,635,560	3,392,059	3,792,524	3,549,965
Weighted average number of common shares and dilutive potential common shares used in diluted EPS (in shares)	168,651,202	168,314,296	168,821,869	169,232,579