Income Taxes Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Examination [Line Items]
Income Tax Examination, Year under Examination		2015	2014	2013	2012	2011	2010
Decrease in Unrecognized Tax Benefits is Reasonably Possible	$ 140	$ 320
Income Taxes Paid	130
Income Tax Examination, Estimate of Cash Payment for Taxes from Settlement with Taxing Authority	$ 30