Cash, Cash Equivalents and Marketable Securities Available-for-Sale (Tables)
 3 Months Ended
Mar. 31, 2015
Cash, Cash Equivalents, and Short-term Investments [Abstract]  
Schedule of available-for-sale securities by major security type and class
The amortized cost, gross unrealized holding gains, gross unrealized holding losses and estimated fair value of available-for-sale securities by major security type and class of security at March 31, 2015 and December 31, 2014 were as follows:
March 31, 2015
 
Amortized Cost
 
Gross Unrealized Gain
 
Gross Unrealized Loss
 
Estimated Fair Value
U.S. Treasury securities
 
$
587.3

 
$
1.2

 
$
(0.1
)
 
$
588.4

U.S. government-sponsored agency securities
 
97.0

 
0.3

 
—

 
97.3

U.S. government-sponsored agency MBS
 
357.6

 
1.3

 
(1.4
)
 
357.5

Non-U.S. government, agency and Supranational securities
 
27.8

 
—

 
—

 
27.8

Corporate debt - global
 
761.6

 
2.4

 
(0.3
)
 
763.7

Asset backed securities
 
144.0

 
0.1

 
(0.1
)
 
144.0

Marketable equity securities
 
335.2

 
642.9

 
(11.0
)
 
967.1

Total available-for-sale marketable securities
 
$
2,310.5

 
$
648.2

 
$
(12.9
)

$
2,945.8

 
December 31, 2014
 
Amortized Cost
 
Gross Unrealized Gain
 
Gross Unrealized Loss
 
Estimated Fair Value
U.S. Treasury securities
 
$
1,044.7

 
$
0.3

 
$
(0.8
)
 
$
1,044.2

U.S. government-sponsored agency securities
 
145.1

 
0.1

 
(0.1
)
 
145.1

U.S. government-sponsored agency MBS
 
531.1

 
1.0

 
(2.7
)
 
529.4

Non-U.S. government, agency and Supranational securities
 
32.4

 
—

 
(0.1
)
 
32.3

Corporate debt - global
 
446.3

 
0.6

 
(1.2
)
 
445.7

Asset backed securities
 
177.3

 
—

 
(0.2
)
 
177.1

Marketable equity securities
 
335.2

 
716.3

 
(0.2
)
 
1,051.3

Total available-for-sale marketable securities
 
$
2,712.1

 
$
718.3


$
(5.3
)

$
3,425.1
Schedule of duration periods of available-for-sale debt securities
Duration periods of available-for-sale debt securities at March 31, 2015 were as follows:
 
 
 
Amortized
Cost
 
Fair
Value
Duration of one year or less
 
$
327.9

 
$
328.2

Duration of one through three years
 
1,547.5

 
1,550.2

Duration of three through five years
 
99.9

 
100.3

Total
 
$
1,975.3

 
$
1,978.7