Annual report pursuant to Section 13 and 15(d)

Quarterly Financial Data (Unaudited) (Details)

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Quarterly Financial Data (Unaudited) (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
3 Months Ended 12 Months Ended
May 31, 2013
Feb. 28, 2013
Nov. 30, 2012
Aug. 31, 2012
May 31, 2012
Feb. 29, 2012
Nov. 30, 2011
Aug. 31, 2011
May 31, 2013
May 31, 2012
May 31, 2011
Quarterly Financial Data (Unaudited) [Abstract]                      
Service revenue $ 575.3 $ 582.4 $ 559.4 $ 568.1 $ 540.7 $ 558.5 $ 535.0 $ 552.0 $ 2,285.2 $ 2,186.2 $ 2,036.2
Interest on funds held for clients 10.0 10.9 10.0 10.1 10.8 11.0 10.7 11.1 41.0 43.6 48.1
Total revenue 585.3 593.3 569.4 578.2 551.5 569.5 545.7 563.1 2,326.2 2,229.8 2,084.3
Operating income 211.8 225.0 230.0 238.0 195.9 210.4 217.9 229.7 904.8 853.9 786.4
Investment income, net 1.4 1.4 1.9 1.9 1.8 1.6 1.5 1.5 6.6 6.4 5.8
Income before income taxes 213.2 226.4 231.9 239.9 197.7 212.0 219.4 231.2 911.4 860.3 792.2
Income taxes 89.7 81.9 84.0 86.8 74.4 76.6 79.0 82.3 342.4 312.3 276.9
Net income 123.5 144.5 147.9 153.1 123.3 135.4 140.4 148.9 569.0 548.0 515.3
Basic earnings per share $ 0.34 $ 0.40 $ 0.41 $ 0.42 $ 0.34 $ 0.37 $ 0.39 $ 0.41 $ 1.56 $ 1.51 $ 1.42
Diluted earnings per share $ 0.34 $ 0.40 $ 0.41 $ 0.42 $ 0.34 $ 0.37 $ 0.39 $ 0.41 $ 1.56 $ 1.51 $ 1.42
Weighted-average common shares outstanding 364.6 363.8 363.6 363.0 362.6 362.5 362.4 362.2 363.8 362.4 361.8
Weighted-average common shares outstanding, assuming dilution 365.9 364.6 364.4 363.8 363.4 363.1 362.8 362.8 364.7 363.0 362.4
Cash dividends per common share $ 0.00 $ 0.66 $ 0.33 $ 0.32 $ 0.32 $ 0.32 $ 0.32 $ 0.31 $ 1.31 $ 1.27 $ 1.24
Total net realized gains $ 0 $ 0.6 $ 0.1 $ 0.2 $ 0.4 $ 0.4 $ 0.1 $ 0.1 $ 0.9 $ 1.0 $ 1.3