Fair Value of Financial Instruments - Gross Transfers Into and Out of Certain Fair Value Levels by Asset Class (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 30	$ 0	$ 30	$ 0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount	189	74	494	206
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount	13	27	69	84
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount	13	27	24	84
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount	159	74	464	206
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount	0	0	45	0
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0		0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount	90		343
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount	13		13
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount	13		13
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount	90		343
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount	0		0
Asset-backed securities | Predecessor
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount		0		0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount		74		154
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount		22		58
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount		22		58
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount		74		154
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount		0		0
Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0		0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount	12		13
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount	0		8
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount	0		8
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount	12		13
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount	0		0
Commercial mortgage-backed securities | Predecessor
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount		0		0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount		0		8
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount		0		6
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount		0		6
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount		0		8
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount		0		0
Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0		0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount	21		72
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount	0		3
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount	0		3
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount	21		72
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount	0		0
Corporates | Predecessor
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount		0		0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount		0		44
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount		5		6
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount		5		6
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount		0		44
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount		0		0
Hybrids
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0		0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount	0		0
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount	0		20
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount	0		0
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount	0		0
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount	0		20
Hybrids | Predecessor
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount		0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount		0
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount		0
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount		0
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount		0
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount		0
Hybrids
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	30		30
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount	30		30
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount	0		25
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount	0		0
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount	0		0
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount	0		25
Hybrids | Predecessor
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount		0		0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount		0		0
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount		0		0
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount		0		0
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount		0		0
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount		$ 0		0
Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0		0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount	36		36
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount	0		0
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount	0		0
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount	36		36
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount	$ 0		$ 0
Residential mortgage-backed securities | Predecessor
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount				0
Fair Value, Assets, Level 1 Or Level 3 To Level 2 Transfers, Amount				0
Fair Value, Assets, Level 2 To Level 1 Or Level 3 Transfers, Amount				14
Fair Value, Assets, Level 1 Or Level 2 To Level 3 Transfers, Amount				14
Fair Value, Assets, Level 3 To Level 1 Or Level 2 Transfers, Amount				0
Fair Value, Assets, Level 2 Or Level 3 From Level 1 Transfers, Amount				$ 0