Net Income Per Share - Computation Of Basic And Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 204.0	$ 211.0	$ 184.5	$ 136.4	$ 190.0	$ 167.3	$ 134.5	$ 97.0	$ 735.9	$ 588.8	$ 418.8
Weighted-average shares outstanding basic (shares)									38.3	37.1	36.9
Basic net income per share (usd per share)	$ 5.26	$ 5.45	$ 4.82	$ 3.62	$ 5.09	$ 4.49	$ 3.64	$ 2.64	$ 19.21	$ 15.87	$ 11.35
Weighted-average shares outstanding basic (shares)									38.3	37.1	36.9
Add: Dilutive potential shares									1.0	0.8	0.8
Weighted-average shares used in computing diluted net income per share (shares)									39.3	37.9	37.7
Diluted net income per share (usd per share)	$ 5.13	$ 5.31	$ 4.71	$ 3.54	$ 4.99	$ 4.40	$ 3.56	$ 2.57	$ 18.73	$ 15.54	$ 11.11