Income Taxes (Effective Income Tax Rate) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended	3 Months Ended			12 Months Ended
	Jul. 10, 2013	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate		35.00%
Total income tax benefit (expense)	$ (1,600)
Successor [Member]
Operating Loss Carryforwards [Line Items]
Income tax benefit (expense) at the federal statutory rate		1,372	33	3	8	635
State income taxes, net of federal income tax effect		124	(4)	0	0	47
State law changes, net of federal income tax effect		4	5	0	0	10
Reduction (increase) in liability for unrecognized tax benefits		1	0	0	0	2
Change in valuation allowance		(911)	(82)	0	(4)	(708)
Other, net		(16)	(8)	(4)	(8)	(31)
Total income tax benefit (expense)		574	(56)	(1)	(4)	(45)
Effective income tax rate		(14.60%)	58.90%	12.50%	17.40%	(2.50%)
Predecessor [Member]
Operating Loss Carryforwards [Line Items]
Income tax benefit (expense) at the federal statutory rate	(155)			212			1,460
State income taxes, net of federal income tax effect	(18)			16			137
State law changes, net of federal income tax effect	0			0			(5)
Reduction (increase) in liability for unrecognized tax benefits	(7)			0			38
Change in valuation allowance	(1,410)			(265)			(1,756)
Other, net	(11)			(1)			(28)
Total income tax benefit (expense)	$ (1,601)			$ (38)			$ (154)
Effective income tax rate	361.40%			6.30%			(3.70%)