Segment and Related Information (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment information
Revenue	$ 1,908.0	$ 1,956.2	$ 4,726.4	$ 4,576.6
Operating Income	606.8	630.0	1,180.1	1,145.4
S&P [Member]
Segment information
Revenue	409.9	417.5	1,333.1	1,223.8
Operating Income	169.1	187.3	572.2	557.5
MH Financial [Member]
Segment information
Revenue	348.5	294.3	1,005.9	866.8
Operating Income	112.6	85.8	306.7	240.3
MHE [Member]
Segment information
Revenue	937.3	1,054.7	1,776.6	1,936.9
Operating Income	314.7	357.5	281.4	347.3
I&M [Member]
Segment information
Revenue	228.5	204.1	657.6	590.4
Operating Income	51.3	43.8	139.1	118.0
Intersegment elimination [Member]
Segment information
Revenue	(16.2)	(14.4)	(46.8)	(41.3)
Total operating segments [Member]
Segment information
Revenue	1,908.0	1,956.2	4,726.4	4,576.6
Operating Income	647.7	674.4	1,299.4	1,263.1
General corporate expense [Member]
Segment information
Operating Income	$ (40.9)	$ (44.4)	$ (119.3)	$ (117.7)