Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at the beginning of the period	$ 767.7	$ 914.8
Other comprehensive income (loss) before reclassifications	(435.6)	337.1
Amounts reclassified from accumulated other comprehensive income	(80.6)	(70.1)
Total other comprehensive income (loss)	(516.2)	267.0
Balance at the end of the period	251.5	1,181.8
Pension Liability
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at the beginning of the period	(13.9)	(15.5)
Other comprehensive income (loss) before reclassifications	0.0	(5.9)
Amounts reclassified from accumulated other comprehensive income	0.0	0.0
Total other comprehensive income (loss)	0.0	(5.9)
Balance at the end of the period	(13.9)	(21.4)
Net Unrealized Gains (Losses) From Marketable Securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at the beginning of the period	271.5	460.9
Other comprehensive income (loss) before reclassifications	(242.8)	(50.6)
Amounts reclassified from accumulated other comprehensive income	6.8	(0.4)
Total other comprehensive income (loss)	(236.0)	(51.0)
Balance at the end of the period	35.5	409.9
Net Unrealized Gains (Losses) From Hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at the beginning of the period	586.4	519.6
Other comprehensive income (loss) before reclassifications	(211.0)	416.8
Amounts reclassified from accumulated other comprehensive income	(87.4)	(69.7)
Total other comprehensive income (loss)	(298.4)	347.1
Balance at the end of the period	288.0	866.7
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at the beginning of the period	(76.3)	(50.2)
Other comprehensive income (loss) before reclassifications	18.2	(23.2)
Amounts reclassified from accumulated other comprehensive income	0.0	0.0
Total other comprehensive income (loss)	18.2	(23.2)
Balance at the end of the period	$ (58.1)	$ (73.4)