Income Taxes - Tax Carryforwards and Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	$ 604	$ 1,070
Tax credit carryforwards	2,916	2,742
Federal [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	361
Tax credit carryforwards	2,585
State [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	153
Tax credit carryforwards	331
Tax credit carryforward with indefinite expiration period	158
Foreign [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	90
Operating loss carryforwards with indefinite expiration period	89
FPL [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	4	0
Tax credit carryforwards	$ 0	$ 0