Income Taxes (Effective Income Tax Rate) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Jul. 10, 2013	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2013
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate				35.00%
Total income tax (expense) benefit			$ (1,600)
Successor [Member]
Operating Loss Carryforwards [Line Items]
Income tax benefit (expense) at the federal statutory rate	$ 33	$ 3		$ 649	$ 1,372	$ 635
State income taxes, net of federal income tax effect	(4)	0		38	124	47
State law changes, net of federal income tax effect	5	0		20	4	10
Reduction (increase) in liability for unrecognized tax benefits	0	0		(4)	1	2
Tax benefit from organizational restructuring	0	0		90	0	0
Change in valuation allowance	(82)	0		(939)	(911)	(708)
Other, net	(8)	(4)		5	(16)	(31)
Total income tax (expense) benefit	$ (56)	$ (1)		$ (141)	$ 574	$ (45)
Effective income tax rate	58.90%	12.50%		7.60%	(14.60%)	(2.50%)
Predecessor [Member]
Operating Loss Carryforwards [Line Items]
Income tax benefit (expense) at the federal statutory rate		$ 212	(155)
State income taxes, net of federal income tax effect		16	(18)
State law changes, net of federal income tax effect		0	0
Reduction (increase) in liability for unrecognized tax benefits		0	(7)
Tax benefit from organizational restructuring		0	0
Change in valuation allowance		(265)	(1,410)
Other, net		(1)	(11)
Total income tax (expense) benefit		$ (38)	$ (1,601)
Effective income tax rate		6.30%	361.40%