Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) shares in Millions, $ in Millions	Total	Common Stock Class A	Common Stock Class B	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjustment Retained Earnings
Balance (in shares) at Dec. 31, 2018		22.0	1.0
Balance at Dec. 31, 2018	$ 1,197.2	$ 0.0	$ 0.1	$ 35.0	$ 0.0	$ 1,162.1	$ 0.9	$ 0.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	56.4					56.4
Issuance of stock in connection with employee stock plans	2.3	$ 2.3
Issuance of restricted stock to employees (in shares)		0.1
Issuance of restricted stock to employees	0.0	$ 0.0
Repurchase of Class A common stock	(3.1)	$ (3.1)
Class B common stock converted to Class A common stock (in shares)		0.2	(0.2)
Class B common stock converted to Class A common stock	0.0
Compensation for stock and stock option issuances and excess tax benefits from option exercises	3.5	$ 10.5		(7.0)
Dividends paid	(6.7)					(6.7)
Balance (in shares) at Mar. 31, 2019		22.3	0.8
Balance at Mar. 31, 2019	1,250.5	$ 9.7	$ 0.1	28.0	0.0	1,212.7
Balance (in shares) at Dec. 31, 2018		22.0	1.0
Balance at Dec. 31, 2018	1,197.2	$ 0.0	$ 0.1	35.0	0.0	1,162.1	0.9	0.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	203.5
Gain (loss) on cash flow hedges, net of tax benefit (expense)	(1.9)
Balance (in shares) at Sep. 30, 2019		22.6	0.6
Balance at Sep. 30, 2019	$ 1,359.2	$ 16.9	$ 0.1	3.4	(1.9)	1,340.7
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Balance (in shares) at Dec. 31, 2018		22.0	1.0
Balance at Dec. 31, 2018	$ 1,197.2	$ 0.0	$ 0.1	35.0	0.0	1,162.1	0.9	0.9
Balance (in shares) at Dec. 31, 2019		22.6	0.6
Balance at Dec. 31, 2019	1,467.7	$ 20.5	$ 0.1	46.0	(0.7)	1,401.8	(4.8)	(4.8)
Balance (in shares) at Mar. 31, 2019		22.3	0.8
Balance at Mar. 31, 2019	1,250.5	$ 9.7	$ 0.1	28.0	0.0	1,212.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	61.9					61.9
Gain (loss) on cash flow hedges, net of tax benefit (expense)	(0.9)				(0.9)
Issuance of stock in connection with employee stock plans (in shares)		0.1
Issuance of stock in connection with employee stock plans	2.8	$ 2.8
Compensation for stock and stock option issuances and excess tax benefits from option exercises	3.9	$ 0.5		3.4
Option premiums paid	(36.5)			(31.4)		(5.1)
Dividends paid	(7.0)					(7.0)
Balance (in shares) at Jun. 30, 2019		22.4	0.8
Balance at Jun. 30, 2019	1,274.7	$ 13.0	$ 0.1	0.0	(0.9)	1,262.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	85.2					85.2
Gain (loss) on cash flow hedges, net of tax benefit (expense)	(1.0)				(1.0)
Issuance of stock in connection with employee stock plans	2.9	$ 2.9
Class B common stock converted to Class A common stock (in shares)		0.2	(0.2)
Class B common stock converted to Class A common stock	0.0
Compensation for stock and stock option issuances and excess tax benefits from option exercises	4.4	$ 1.0		3.4
Dividends paid	(7.0)					(7.0)
Balance (in shares) at Sep. 30, 2019		22.6	0.6
Balance at Sep. 30, 2019	1,359.2	$ 16.9	$ 0.1	3.4	(1.9)	1,340.7
Balance (in shares) at Dec. 31, 2019		22.6	0.6
Balance at Dec. 31, 2019	1,467.7	$ 20.5	$ 0.1	46.0	(0.7)	1,401.8	(4.8)	(4.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	46.2					46.2
Gain (loss) on cash flow hedges, net of tax benefit (expense)	(5.1)				(5.1)
Issuance of stock in connection with employee stock plans	2.6	$ 2.6
Issuance of restricted stock to employees (in shares)		0.1
Issuance of restricted stock to employees	0.0	$ 0.0
Repurchase of Class A common stock (in shares)		(0.5)
Repurchase of Class A common stock	(48.2)	$ (32.0)		(16.2)
Compensation for stock and stock option issuances and excess tax benefits from option exercises	5.1	$ 8.9		(3.8)
Dividends paid	(7.0)					(7.0)
Balance (in shares) at Mar. 31, 2020		22.2	0.6
Balance at Mar. 31, 2020	$ 1,456.5	$ 0.0	$ 0.1	26.0	(5.8)	1,436.2
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Balance (in shares) at Dec. 31, 2019		22.6	0.6
Balance at Dec. 31, 2019	$ 1,467.7	$ 20.5	$ 0.1	46.0	(0.7)	1,401.8	$ (4.8)	$ (4.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	282.7
Gain (loss) on cash flow hedges, net of tax benefit (expense)	(6.1)
Balance (in shares) at Sep. 30, 2020		22.4	0.4
Balance at Sep. 30, 2020	1,694.4	$ 5.9	$ 0.1	36.4	(6.8)	1,658.8
Balance (in shares) at Mar. 31, 2020		22.2	0.6
Balance at Mar. 31, 2020	1,456.5	$ 0.0	$ 0.1	26.0	(5.8)	1,436.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	77.7					77.7
Gain (loss) on cash flow hedges, net of tax benefit (expense)	(1.4)				(1.4)
Issuance of stock in connection with employee stock plans	3.5	3.5
Repurchase of Class A common stock	(2.3)	$ (2.3)
Class B common stock converted to Class A common stock (in shares)		0.2	(0.2)
Class B common stock converted to Class A common stock	0.0	$ 0.0	$ 0.0
Compensation for stock and stock option issuances and excess tax benefits from option exercises	5.0	$ 0.9		4.1
Dividends paid	(6.8)					(6.8)
Balance (in shares) at Jun. 30, 2020		22.4	0.4
Balance at Jun. 30, 2020	1,532.2	$ 2.1	$ 0.1	30.1	(7.2)	1,507.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	158.8					158.8
Gain (loss) on cash flow hedges, net of tax benefit (expense)	0.4				0.4
Issuance of stock in connection with employee stock plans	3.1	3.1
Repurchase of Class A common stock	(0.1)	(0.1)
Compensation for stock and stock option issuances and excess tax benefits from option exercises	7.1	$ 0.8		6.3
Dividends paid	(7.1)					(7.1)
Balance (in shares) at Sep. 30, 2020		22.4	0.4
Balance at Sep. 30, 2020	$ 1,694.4	$ 5.9	$ 0.1	$ 36.4	$ (6.8)	$ 1,658.8