Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 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			(395.2)	302.0
Amounts reclassified from accumulated other comprehensive income			(121.5)	(158.7)
Total other comprehensive income (loss)	$ (0.5)	$ (123.7)	(516.7)	143.3
Balance at the end of the period	251.0	1,058.1	251.0	1,058.1
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	(7.6)
Amounts reclassified from accumulated other comprehensive income			0.0	0.0
Total other comprehensive income (loss)			0.0	(7.6)
Balance at the end of the period	(13.9)	(23.1)	(13.9)	(23.1)
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			(226.0)	(5.2)
Amounts reclassified from accumulated other comprehensive income			26.4	0.5
Total other comprehensive income (loss)			(199.6)	(4.7)
Balance at the end of the period	71.9	456.2	71.9	456.2
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			(171.9)	322.5
Amounts reclassified from accumulated other comprehensive income			(147.9)	(159.2)
Total other comprehensive income (loss)			(319.8)	163.3
Balance at the end of the period	266.6	682.9	266.6	682.9
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			2.7	(7.7)
Amounts reclassified from accumulated other comprehensive income			0.0	0.0
Total other comprehensive income (loss)			2.7	(7.7)
Balance at the end of the period	$ (73.6)	$ (57.9)	$ (73.6)	$ (57.9)