Stockholders' Equity - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Numerator used in basic and diluted EPS:
Net income	$ 133.0	$ 108.8	$ 555.1
Denominator:
Weighted average number of common shares used in basic EPS (in shares)	165,043,047	166,442,991
Effect of dilutive shares:
Potential common shares issuable from stock options and stock awards (in shares)	3,949,488	3,707,871
Weighted average number of common shares and dilutive potential common shares used in diluted EPS (in shares)	168,992,535	170,150,862