Accumulated Other Comprehensive Income (Summary of Components) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of accumulated other comprehensive income (loss), net of tax
Balance at the beginning of the period	$ 94.0	$ (49.7)
Other comprehensive income (loss) before reclassifications	842.1	152.8
Amount reclassified from accumulated other comprehensive income	(21.3)	(9.1)
Total other comprehensive income	820.8	143.7	12.6
Balance at the end of the period	914.8	94.0	(49.7)
Pension Liability
Summary of accumulated other comprehensive income (loss), net of tax
Balance at the beginning of the period	(6.9)	(10.1)
Other comprehensive income (loss) before reclassifications	(8.6)	3.2
Amount reclassified from accumulated other comprehensive income	0	0
Total other comprehensive income	(8.6)	3.2
Balance at the end of the period	(15.5)	(6.9)
Net Unrealized Gains (Losses) From Marketable Securities
Summary of accumulated other comprehensive income (loss), net of tax
Balance at the beginning of the period	137.3	4.2
Other comprehensive income (loss) before reclassifications	320.1	128.0
Amount reclassified from accumulated other comprehensive income	3.5	5.1
Total other comprehensive income	323.6	133.1
Balance at the end of the period	460.9	137.3
Net Unrealized Gains (Losses) From Hedges
Summary of accumulated other comprehensive income (loss), net of tax
Balance at the beginning of the period	(36.0)	(16.0)
Other comprehensive income (loss) before reclassifications	580.4	(5.8)
Amount reclassified from accumulated other comprehensive income	(24.8)	(14.2)
Total other comprehensive income	555.6	(20.0)
Balance at the end of the period	519.6	(36.0)
Foreign Currency Translation Adjustment
Summary of accumulated other comprehensive income (loss), net of tax
Balance at the beginning of the period	(0.4)	(27.8)
Other comprehensive income (loss) before reclassifications	(49.8)	27.4
Amount reclassified from accumulated other comprehensive income	0	0
Total other comprehensive income	(49.8)	27.4
Balance at the end of the period	$ (50.2)	$ (0.4)