Stock-Based Compensation - Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax unrecognized compensation cost	$ 145.1
Weighted average period for recognizing compensation cost (in years)	1 year 2 months
Closing stock price (in dollars per share)	$ 62.96
In the money options exercisable (in shares)	2,700,000
Prolexic
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options acquired in acquisition	328,000
Capitalized Internal Use Software
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional stock based compensation	10.3	8.1	7.7
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of stock options granted (in dollars per share)	$ 49.67	$ 14.17	$ 25.20
Total pre-tax intrinsic value of options exercised	45.8	47.2	47.9
Total fair value of vested options	16.9	12.4	16.6
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)	$ 12.64	$ 11.34	$ 8.71
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)	$ 54.06
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%
Number of years from date of grant DSUs are fully vested (in years)		2 years
Director's minimum period of service before vesting accelerates (in years)	1 year
Total pre-tax intrinsic value end of year	1.4	3.8	2.3
Total fair value of vested and distributed	0.8	1.5	2.4
Unvested deferred stock units (in shares)	57,000
Aggregate intrinsic value	3.6
Weighted average contractual remaining life (in years)	6 years 2 months
Deferred Stock Units | First Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage		50.00%
Deferred Stock Units | Remaining After First Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage		50.00%
Deferred Stock Units | Equal Installments After First Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage		12.50%
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)	$ 59.72
Total pre-tax intrinsic value end of year	145.6	117.5	98.3
Total fair value of vested and distributed	86.9	89.2	71.4
Aggregate intrinsic value	285.7
Weighted average contractual remaining life (in years)	5 years 7 months
Each RSU receives this number of shares of common stock upon vesting (in shares)	1
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)	4,500,000
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)	8,000,000
Common stock available for grant (in shares)	10,200,000
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
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)	700,000	600,000	700,000
Weighted average purchase price (in dollars per share)	$ 41.76	$ 34.26	$ 24.76
Total cash proceeds from shares purchased	29.3	22.1	16.8
Amount withheld from employees for future purchases	$ 3.0