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Cash, Cash Equivalents & Investments (Details) (USD $) In Millions	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010	Dec. 31, 2009
Cash, amortized cost	$ 20.3	$ 20.1
Cash, gross unrealized gains	0	0
Cash, gross unrealized losses	0	0
Cash, fair value	20.3	20.1
Cash equivalents, amortized cost	361.7	259.7
Cash equivalents, gross unrealized gains	0	0
Cash equivalents, gross unrealized losses	0	0
Cash equivalents, fair value	361.7	259.7
Total cash and cash equivalents, amortized cost	382.0	279.8
Total cash and cash equivalents, gross unrealized gains	0	0
Total cash and cash equivalents, gross unrealized losses	0	0
Total cash and cash equivalents, fair value	382.0	279.8	420.0	221.4
Amortized cost	1,798.9
Available for sale investments, fair value	1,801.7	1,588.8
Available for sale investments, fair value	1,800.5
Total cash, cash equivalents and available-for-sale investments, amortized cost	1,819.9	1,610.0
Total cash, cash equivalents and available-for-sale investments, total gross unrealized gains	6.3	4.6
Total cash, cash equivalents and available-for-sale investments, total gross unrealized losses	(4.2)	(5.7)
Total cash, cash equivalents and available-for-sale investments, fair value	1,822.0	1,608.9
Accumulated net realized gains on available-for-sale securities, net of tax	2.1	1.3
Realized gains recognized on the sale of investments	2.4
Short-Term [Member]
Available for sale investments, amortized cost	609.9	628.6
Available for sale investments, gross unrealized gains	2.0	2.0
Available for sale investments, gross unrealized losses	0	0
Available for sale investments, fair value	611.9	630.6
Short-Term [Member] \| Commercial Paper [Member]
Available for sale investments, amortized cost	90.3	79.0
Available for sale investments, gross unrealized gains	0	0
Available for sale investments, gross unrealized losses	0	0
Available for sale investments, fair value	90.3	79.0
Short-Term [Member] \| Municipal Notes [Member]
Available for sale investments, amortized cost	108.4	111.8
Available for sale investments, gross unrealized gains	0.3	0.4
Available for sale investments, gross unrealized losses	0	0
Available for sale investments, fair value	108.7	112.2
Short-Term [Member] \| U.S. Corporate Debt [Member]
Available for sale investments, amortized cost		174.1
Available for sale investments, gross unrealized gains		1.3
Available for sale investments, gross unrealized losses		0
Available for sale investments, fair value		175.4
Short-Term [Member] \| U.S. Corporate Debt & Equity Securities [Member]
Available for sale investments, amortized cost	170.0
Available for sale investments, gross unrealized gains	1.4
Available for sale investments, gross unrealized losses	0
Available for sale investments, fair value	171.4
Short-Term [Member] \| U.S. Treasuries [Member]
Available for sale investments, amortized cost	106.8	76.3
Available for sale investments, gross unrealized gains	0.1	0
Available for sale investments, gross unrealized losses	0	0
Available for sale investments, fair value	106.9	76.3
Short-Term [Member] \| U.S. Government Agencies [Member]
Available for sale investments, amortized cost	134.4	187.4
Available for sale investments, gross unrealized gains	0.2	0.3
Available for sale investments, gross unrealized losses	0	0
Available for sale investments, fair value	134.6	187.7
Long-Term [Member]
Available for sale investments, amortized cost	828.0	701.6
Available for sale investments, gross unrealized gains	4.3	2.6
Available for sale investments, gross unrealized losses	(4.2)	(5.7)
Available for sale investments, fair value	828.1	698.5
Long-Term [Member] \| Municipal Notes [Member]
Available for sale investments, amortized cost	142.8	143.4
Available for sale investments, gross unrealized gains	0.6	0.3
Available for sale investments, gross unrealized losses	(3.9)	(4.3)
Available for sale investments, fair value	139.5	139.4
Long-Term [Member] \| U.S. Corporate Debt [Member]
Available for sale investments, amortized cost	380.9	300.4
Available for sale investments, gross unrealized gains	2.4	1.8
Available for sale investments, gross unrealized losses	(0.3)	(0.9)
Available for sale investments, fair value	383.0	301.3
Long-Term [Member] \| U.S. Treasuries [Member]
Available for sale investments, amortized cost	18.4	39.9
Available for sale investments, gross unrealized gains	0.1	0.1
Available for sale investments, gross unrealized losses	0	0
Available for sale investments, fair value	18.5	40.0
Long-Term [Member] \| U.S. Government Agencies [Member]
Available for sale investments, amortized cost	253.2	196.7
Available for sale investments, gross unrealized gains	0.9	0.3
Available for sale investments, gross unrealized losses	0	(0.4)
Available for sale investments, fair value	254.1	196.6
Long-Term [Member] \| Non-U.S. Government Securities [Member]
Available for sale investments, amortized cost	32.7	21.2
Available for sale investments, gross unrealized gains	0.3	0.1
Available for sale investments, gross unrealized losses	0	(0.1)
Available for sale investments, fair value	33.0	21.2
Mature in Less Than One Year [Member]
Amortized cost	970.9
Available for sale investments, fair value	972.4
Mature in One to Five Years [Member]
Amortized cost	805.5
Available for sale investments, fair value	809.4
Mature in More Than Five Years [Member]
Amortized cost	22.5
Available for sale investments, fair value	18.7
Commercial Paper [Member]
Available for sale investments, fair value	114.1	122.5
Municipal Notes [Member]
Available for sale investments, fair value	248.2	251.7
U.S. Corporate Debt [Member]
Available for sale investments, fair value		476.8
U.S. Corporate Debt & Equity Securities [Member]
Available for sale investments, fair value	554.4
U.S. Treasuries [Member]
Available for sale investments, fair value	125.4	116.3
U.S. Government Agencies [Member]
Available for sale investments, fair value	388.7	389.2
Non-U.S. Government Securities [Member]
Available for sale investments, fair value	$ 33.0	$ 21.1