Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details 6) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense		$ 1,111	$ 1,386	$ 1,116
Other Assets		(2,010)	(1,992)
Long-term Debt		$ (5,124)	(5,140)
Accounting Standards Update 2014-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense	$ 45
Long-term Debt			54
Adjustments for New Accounting Pronouncement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Assets			$ 54