Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance of unrecognized tax benefits	$ 426	$ 503
Additions (reductions) for tax positions of prior years	(5)	(76)
Settlements	(417)	(1)
Lapse of statute of limitations	0	(1)
Unrecognized deferred tax benefits from acquisitions	0	1
Ending balance of unrecognized tax benefits	4	426
Federal
Income Tax Examination [Line Items]
Unrecognized tax benefits that would have impacted effective rate if recognized	0	422
State
Income Tax Examination [Line Items]
Unrecognized tax benefits that would have impacted effective rate if recognized	$ 3	$ 3