Accumulated Other Comprehensive Income (Summary of Components) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of accumulated other comprehensive income (loss), net of tax
Balance at the beginning of the period	$ 914.8	$ 94.0
Other comprehensive income (loss) before reclassifications	188.6	842.1
Amount reclassified from accumulated other comprehensive income	(335.7)	(21.3)
Total other comprehensive income (loss)	(147.1)	820.8	$ 143.7
Balance at the end of the period	767.7	914.8	94.0
Pension Liability
Summary of accumulated other comprehensive income (loss), net of tax
Balance at the beginning of the period	(15.5)	(6.9)
Other comprehensive income (loss) before reclassifications	1.6	(8.6)
Amount reclassified from accumulated other comprehensive income	0.0	0.0
Total other comprehensive income (loss)	1.6	(8.6)
Balance at the end of the period	(13.9)	(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	460.9	137.3
Other comprehensive income (loss) before reclassifications	(204.6)	320.1
Amount reclassified from accumulated other comprehensive income	15.2	3.5
Total other comprehensive income (loss)	(189.4)	323.6
Balance at the end of the period	271.5	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	519.6	(36.0)
Other comprehensive income (loss) before reclassifications	417.7	580.4
Amount reclassified from accumulated other comprehensive income	(350.9)	(24.8)
Total other comprehensive income (loss)	66.8	555.6
Balance at the end of the period	586.4	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	(50.2)	(0.4)
Other comprehensive income (loss) before reclassifications	(26.1)	(49.8)
Amount reclassified from accumulated other comprehensive income	0.0	0.0
Total other comprehensive income (loss)	(26.1)	(49.8)
Balance at the end of the period	$ (76.3)	$ (50.2)	$ (0.4)