Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Cash and cash equivalents	$ 1,160.7	$ 1,014.3
Marketable securities	10.4	15.5
Derivative instruments	9.3	21.5
Benefit trust assets	169.5	172.5
Total	1,349.9	1,223.8
Derivative instruments	22.2	2.6
Benefit trust liabilities	178.3	195.8
Total	200.5	198.4
Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	1,160.7	1,014.3
Marketable securities	10.4	15.5
Derivative instruments	0	0
Benefit trust assets	13.3	17.3
Total	1,184.4	1,047.1
Derivative instruments	0	0
Benefit trust liabilities	15.9	17.4
Total	15.9	17.4
Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents	0	0
Marketable securities	0	0
Derivative instruments	9.3	21.5
Benefit trust assets	156.2	155.2
Total	165.5	176.7
Derivative instruments	22.2	2.6
Benefit trust liabilities	162.4	178.4
Total	184.6	181.0
Fair Value, Inputs, Level 3 [Member]
Cash and cash equivalents	0	0
Marketable securities	0	0
Derivative instruments	0	0
Benefit trust assets	0	0
Total	0	0
Derivative instruments	0	0
Benefit trust liabilities	0	0
Total	$ 0	$ 0