Fair Value Measurements - Significant Unobservable Inputs - Securities (Details)	3 Months Ended		4 Months Ended	5 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	May 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	$ 14,072,000,000		$ 14,072,000,000		$ 14,072,000,000		$ 22,799,000,000
Fair value assets level 1 to level 2 Transfer amount	13,000,000	$ 163,000,000	71,000,000	$ 283,000,000		$ 657,000,000
Fair value assets level 2 to level 1 transfer amount	0	$ 0	0	$ 0		$ 0
RMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	1,030,000,000		1,030,000,000		1,030,000,000		1,427,000,000
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	723,000,000		723,000,000		723,000,000		1,266,000,000
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	7,910,000,000		7,910,000,000		7,910,000,000		14,038,000,000
CMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	1,398,000,000		1,398,000,000		1,398,000,000		2,084,000,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	101,000,000		101,000,000		101,000,000		284,000,000
Fair Value, Inputs, Level 1 [Member] | RMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	0		0		0		0
Fair Value, Inputs, Level 1 [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	0		0		0		0
Fair Value, Inputs, Level 1 [Member] | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	0		0		0		0
Fair Value, Inputs, Level 1 [Member] | CMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	0		0		0		0
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	1,020,000,000		1,020,000,000		$ 1,020,000,000		1,286,000,000
Fair Value, Inputs, Level 3 [Member] | Credit Standing Adjustment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Decrease
Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Credit Standing Adjustment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					0.04%
Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Credit Standing Adjustment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					0.27%
Fair Value, Inputs, Level 3 [Member] | RMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	471,000,000		471,000,000		$ 471,000,000		692,000,000
Fair Value, Inputs, Level 3 [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	12,000,000		12,000,000		12,000,000		38,000,000
Fair Value, Inputs, Level 3 [Member] | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	357,000,000		357,000,000		357,000,000		443,000,000
Fair Value, Inputs, Level 3 [Member] | CMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	3,000,000		3,000,000		3,000,000		26,000,000
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	471,000,000		471,000,000		471,000,000		692,000,000
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	4,000,000		4,000,000		$ 4,000,000		$ 22,000,000
Fair Value Measurements, Sensitivity Analysis, Description					Decrease		Decrease
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	157,000,000		157,000,000		$ 157,000,000		$ 190,000,000
Fair Value Measurements, Sensitivity Analysis, Description					Decrease		Decrease
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	3,000,000		3,000,000		$ 3,000,000		$ 15,000,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	12,951,000,000		12,951,000,000		12,951,000,000		21,229,000,000
Fair Value, Inputs, Level 2 [Member] | RMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	559,000,000		559,000,000		559,000,000		735,000,000
Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	711,000,000		711,000,000		711,000,000		1,228,000,000
Fair Value, Inputs, Level 2 [Member] | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	7,553,000,000		7,553,000,000		7,553,000,000		13,595,000,000
Fair Value, Inputs, Level 2 [Member] | CMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	$ 1,395,000,000		$ 1,395,000,000		$ 1,395,000,000		$ 2,058,000,000
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Withdrawal Utilization [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Increase		Increase
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Withdrawal Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Increase		Increase
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Lapse Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Decrease		Decrease
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Reset Elections [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Increase		Increase
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Withdrawal Utilization [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					15.00%		15.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Withdrawal Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					0.00%		0.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Lapse Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					0.00%		0.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Reset Elections [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					30.00%		30.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Withdrawal Utilization [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					100.00%		100.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Withdrawal Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					8.00%		8.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Lapse Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					40.00%		40.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Reset Elections [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					75.00%		75.00%
Other Contract [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Volatility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Increase		Increase
Other Contract [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Equity Volatility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					8.00%		7.00%
Other Contract [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Equity Volatility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range					30.00%		30.00%
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Decrease		Decrease
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.07		0.07		0.07		0.24
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	3.82		3.82		3.82		4.63
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.73		0.73		0.73		0.77
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	1.60		1.60		1.60		1.92
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	1.60		1.60		1.60		2.50
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	1.60		1.60		1.60		2.28
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	1		1		1		1.03
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	7.81		7.81		7.81		10
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	3.04		3.04		3.04		3.55
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Decrease		Decrease
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.09		0.09		0.09		0.09
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	18.16		18.16		18.16		18.16
Measurement Input, Credit Spread [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	2.40		2.40		2.40		4.57
Measurement Input, Constant Prepayment Rate [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Decrease [5]		Decrease [5]
Measurement Input, Constant Prepayment Rate [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0		0		0		0
Measurement Input, Constant Prepayment Rate [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.13		0.13		0.13		0.25
Measurement Input, Constant Prepayment Rate [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.06		0.06		0.06		0.06
Measurement Input, Default Rate [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Decrease		Decrease
Measurement Input, Default Rate [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.02		0.02		0.02		0
Measurement Input, Default Rate [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.08		0.08		0.08		0.07
Measurement Input, Default Rate [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.03		0.03		0.03		0.04
Measurement Input, Loss Severity [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description					Decrease		Decrease
Measurement Input, Loss Severity [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0		0		0		0
Measurement Input, Loss Severity [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	1		1		1		1
Measurement Input, Loss Severity [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.58		0.58		0.58		0.65