Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from the adoption of new accounting standards		$ 190
Unrealized gain of investment	$ 45
Galapagos
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unrealized gain of investment	278
Difference Between Revenue Guidance In Effect Before And After Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues	$ 5
Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from the adoption of new accounting standards		(293)	$ (293)
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from the adoption of new accounting standards		(293)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from the adoption of new accounting standards		483
Retained Earnings | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from the adoption of new accounting standards		$ 293