Fair Value Measurements (Schedule of Company's Fair Value Hierarchy) (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Available for sale investments, fair value	$ 1,801.7	$ 1,588.8
Foreign currency derivatives, assets		0.2
Total assets measured at fair value	1,801.7	1,589.0
Foreign currency derivatives, liabilities	2.2	2.1
Total liabilities measured at fair value	2.2	2.1
Money Market Funds [Member]
Available for sale investments, fair value	337.9	211.2
Money Market Funds [Member] | Level 1 [Member]
Available for sale investments, fair value	337.9	211.2
Money Market Funds [Member] | Level 2 [Member]
Available for sale investments, fair value	0	0
Money Market Funds [Member] | Level 3 [Member]
Available for sale investments, fair value	0	0
U.S. Treasuries [Member]
Available for sale investments, fair value	125.4	116.3
U.S. Treasuries [Member] | Level 1 [Member]
Available for sale investments, fair value	125.4	116.3
U.S. Treasuries [Member] | Level 2 [Member]
Available for sale investments, fair value	0	0
U.S. Treasuries [Member] | Level 3 [Member]
Available for sale investments, fair value	0	0
Commercial Paper [Member]
Available for sale investments, fair value	114.1	122.5
Commercial Paper [Member] | Level 1 [Member]
Available for sale investments, fair value	0	0
Commercial Paper [Member] | Level 2 [Member]
Available for sale investments, fair value	114.1	122.5
Commercial Paper [Member] | Level 3 [Member]
Available for sale investments, fair value	0	0
U.S. Corporate Debt [Member]
Available for sale investments, fair value		476.8
U.S. Corporate Debt [Member] | Level 1 [Member]
Available for sale investments, fair value		0
U.S. Corporate Debt [Member] | Level 2 [Member]
Available for sale investments, fair value		476.8
U.S. Corporate Debt [Member] | Level 3 [Member]
Available for sale investments, fair value		0
U.S. Corporate Debt & Equity Securities [Member]
Available for sale investments, fair value	554.4
U.S. Corporate Debt & Equity Securities [Member] | Level 1 [Member]
Available for sale investments, fair value	1.2
U.S. Corporate Debt & Equity Securities [Member] | Level 2 [Member]
Available for sale investments, fair value	553.2
U.S. Corporate Debt & Equity Securities [Member] | Level 3 [Member]
Available for sale investments, fair value	0
U.S. Government Agencies [Member]
Available for sale investments, fair value	388.7	389.2
U.S. Government Agencies [Member] | Level 1 [Member]
Available for sale investments, fair value	0	0
U.S. Government Agencies [Member] | Level 2 [Member]
Available for sale investments, fair value	388.7	389.2
U.S. Government Agencies [Member] | Level 3 [Member]
Available for sale investments, fair value	0	0
Non-U.S. Government Securities [Member]
Available for sale investments, fair value	33.0	21.1
Non-U.S. Government Securities [Member] | Level 1 [Member]
Available for sale investments, fair value	0	0
Non-U.S. Government Securities [Member] | Level 2 [Member]
Available for sale investments, fair value	33.0	21.1
Non-U.S. Government Securities [Member] | Level 3 [Member]
Available for sale investments, fair value	0	0
Municipal Notes [Member]
Available for sale investments, fair value	248.2	251.7
Municipal Notes [Member] | Level 1 [Member]
Available for sale investments, fair value	0	0
Municipal Notes [Member] | Level 2 [Member]
Available for sale investments, fair value	229.5	233.1
Municipal Notes [Member] | Level 3 [Member]
Available for sale investments, fair value	18.7	18.6
Level 1 [Member]
Available for sale investments, fair value	464.5	327.5
Foreign currency derivatives, assets		0
Total assets measured at fair value	464.5	327.5
Foreign currency derivatives, liabilities	0	0
Total liabilities measured at fair value	0	0
Level 2 [Member]
Available for sale investments, fair value	1,318.5	1,242.7
Foreign currency derivatives, assets		0.2
Total assets measured at fair value	1,318.5	1,242.9
Foreign currency derivatives, liabilities	2.2	2.1
Total liabilities measured at fair value	2.2	2.1
Level 3 [Member]
Available for sale investments, fair value	18.7	18.6
Foreign currency derivatives, assets		0
Total assets measured at fair value	18.7	18.6
Foreign currency derivatives, liabilities	0	0
Total liabilities measured at fair value	$ 0	$ 0