Financial Instruments - Summary Of Cash And Available-For-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,850.0	$ 3,353.4
Gross Unrealized Gains	1.3	1.4
Gross Unrealized Losses	(13.4)	(7.0)
Fair Value	4,837.9	3,347.8
Cash and Cash Equivalents	1,036.6	714.6	$ 600.3	$ 782.1
Short-term Investments	1,518.0	845.2
Long-term Investments	2,283.3	1,788.0
Cash
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	227.7	202.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	227.7	202.6
Cash and Cash Equivalents	227.7	202.6
Short-term Investments	0.0	0.0
Long-term Investments	0.0	0.0
Level 1
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,238.3	684.2
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(2.0)	(1.8)
Fair Value	1,236.4	682.4
Cash and Cash Equivalents	770.3	481.2
Short-term Investments	168.4	52.4
Long-term Investments	297.7	148.8
Level 1 | Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	612.4	430.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	612.4	430.6
Cash and Cash Equivalents	612.4	430.6
Short-term Investments	0.0	0.0
Long-term Investments	0.0	0.0
Level 1 | U.S. treasuries & corporate equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	625.9	253.6
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(2.0)	(1.8)
Fair Value	624.0	251.8
Cash and Cash Equivalents	157.9	50.6
Short-term Investments	168.4	52.4
Long-term Investments	297.7	148.8
Level 2
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,384.0	2,466.6
Gross Unrealized Gains	1.2	1.4
Gross Unrealized Losses	(11.4)	(5.2)
Fair Value	3,373.8	2,462.8
Cash and Cash Equivalents	38.6	30.8
Short-term Investments	1,349.6	792.8
Long-term Investments	1,985.6	1,639.2
Level 2 | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	139.6	76.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	139.6	76.4
Cash and Cash Equivalents	31.1	3.8
Short-term Investments	108.5	72.6
Long-term Investments	0.0	0.0
Level 2 | Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,471.8	1,131.0
Gross Unrealized Gains	0.7	0.8
Gross Unrealized Losses	(5.0)	(3.0)
Fair Value	1,467.5	1,128.8
Cash and Cash Equivalents	2.9	0.0
Short-term Investments	555.4	384.5
Long-term Investments	909.2	744.3
Level 2 | U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	938.7	618.5
Gross Unrealized Gains	0.5	0.0
Gross Unrealized Losses	(2.9)	(1.5)
Fair Value	936.3	617.0
Cash and Cash Equivalents	0.0	27.0
Short-term Investments	342.7	194.8
Long-term Investments	593.6	395.2
Level 2 | Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18.5	28.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.1)
Fair Value	18.5	28.7
Cash and Cash Equivalents	0.0	0.0
Short-term Investments	16.0	10.3
Long-term Investments	2.5	18.4
Level 2 | Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	815.4	611.9
Gross Unrealized Gains	0.0	0.6
Gross Unrealized Losses	(3.5)	(0.6)
Fair Value	811.9	611.9
Cash and Cash Equivalents	4.6	0.0
Short-term Investments	327.0	130.6
Long-term Investments	$ 480.3	$ 481.3