Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax unrecognized compensation cost	$ 267.6
Weighted average period for recognizing compensation cost (in years)	1 year 8 months 12 days
Closing stock price (in dollars per share)	$ 86.38
In the money options exercisable (in shares)	20,000
Capitalized Internal Use Software
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional stock based compensation	$ 30.6	$ 25.2	$ 17.5
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
Expiration period	10 years
Total pre-tax intrinsic value of options exercised	$ 3.9	$ 8.2	12.3
Total fair value of vested options			$ 1.2
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of equity instruments other than options granted (in dollars per share)	$ 20.90	$ 15.29	$ 13.60
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of equity instruments other than options granted (in dollars per share)	$ 76.62
Each DSU receives this number of shares of common stock upon vesting (in whole numbers)	1
Holder elect to defer vested shares period, minimum (in years)	1 year
Holder elect to defer vested shares period, maximum (in years)	10 years
The amount typically vested by anniversary grant date (percentage)	100.00%
Director's minimum period of service before vesting accelerates (in years)	1 year
Total pre-tax intrinsic value end of year	$ 7.7	$ 3.0	$ 1.5
Total fair value of vested and distributed	$ 4.9	1.8	1.7
Unvested deferred stock units (in shares)	25,000
Aggregate intrinsic value	$ 2.2
Weighted average contractual remaining life (in years)	4 months 24 days
Granted (in shares)	25,000
Deferred Stock Units | Director Vesting Acceleration
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	100.00%
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of equity instruments other than options granted (in dollars per share)	$ 72.30
Total pre-tax intrinsic value end of year	$ 189.4	173.6	168.6
Total fair value of vested and distributed	195.5	$ 178.3	$ 173.6
Aggregate intrinsic value	$ 438.4
Weighted average contractual remaining life (in years)	1 year 7 months 24 days
Each RSU receives this number of shares of common stock upon vesting	1
Granted (in shares)	3,201,000
Forfeited (in shares)	654,000
Restricted stock unit vesting provision, minimum (in years)	3 years
Restricted stock unit vesting provision, maximum (in years)	4 years
Number of equity instruments other than options unvested (in shares)	5,100,000
2009 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for grant (in shares)	3,800,000
2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock approved for issuance under plan (in shares)	21,500,000
Common stock available for grant (in shares)	8,800,000
1999 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum amount of shares available for issuance (in shares)	1,500,000
Aggregate amount of shares available, maximum (in shares)	20,000,000.0
Discount on fair market value for purchase of stock (in percentage)	15.00%
Share purchase interval term (in months)	6 months
Issuance of common stock under employee stock purchase plan (in shares)	900,000	1,000,000.0	1,100,000
Weighted average purchase price (in dollars per share)	$ 61.04	$ 52.04	$ 40.18
Total cash proceeds from shares purchased	$ 53.7	$ 50.7	$ 42.3
Amount withheld from employees for future purchases	$ 5.2