Fair Value Measurements - Financial Assets and Financial Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Financial assets at fair value:
Fixed maturities	$ 23,593	$ 23,138
Equity securities	560	588
Short-term investments	220	199
Financial liabilities at fair value:
Unfunded Commitments	$ 65
Minimum [Member]
Financial liabilities at fair value:
Redemption Notice Period	45 days
Maximum [Member]
Financial liabilities at fair value:
Redemption Notice Period	90 days
Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	$ 23,593	23,138
Equity securities	492	588
Subtotal	24,085	23,726
Short-term investments	220	199
Total financial assets at fair value, excluding separate accounts	25,021	24,704
Real estate funds priced at NAV as a practical expedient	263
Financial liabilities at fair value:
Total financial liabilities at fair value	$ 707	787
Redemption Frequency	Quarterly redemption frequency
Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	$ 32	2
Financial liabilities at fair value:
Derivative liabilities	54	25
Guaranteed Minimum Income Benefit [Member]
Financial assets at fair value:
Derivative assets	684	777
Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	684	777
Financial liabilities at fair value:
Derivative liabilities	653	762
Federal government and agency [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	780	779
State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	1,045	1,287
Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	2,455	2,487
Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	18,785	18,088
Mortgage and other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	528	497
Fair Value Inputs Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	271	253
Equity securities	391	412
Subtotal	662	665
Short-term investments	0	0
Total financial assets at fair value, excluding separate accounts	662	665
Financial liabilities at fair value:
Total financial liabilities at fair value	0	0
Fair Value Inputs Level 1 [Member] | Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	0	0
Financial liabilities at fair value:
Derivative liabilities	0	0
Fair Value Inputs Level 1 [Member] | Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	0	0
Financial liabilities at fair value:
Derivative liabilities	0	0
Fair Value Inputs Level 1 [Member] | Federal government and agency [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	271	253
Fair Value Inputs Level 1 [Member] | State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 1 [Member] | Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 1 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 1 [Member] | Mortgage and other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	22,827	22,256
Equity securities	68	73
Subtotal	22,895	22,329
Short-term investments	220	199
Total financial assets at fair value, excluding separate accounts	23,147	22,530
Financial liabilities at fair value:
Total financial liabilities at fair value	54	25
Fair Value Inputs Level 2 [Member] | Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	32	2
Financial liabilities at fair value:
Derivative liabilities	54	25
Fair Value Inputs Level 2 [Member] | Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	0	0
Financial liabilities at fair value:
Derivative liabilities	0	0
Fair Value Inputs Level 2 [Member] | Federal government and agency [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	509	526
Fair Value Inputs Level 2 [Member] | State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	1,045	1,287
Fair Value Inputs Level 2 [Member] | Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	2,411	2,442
Fair Value Inputs Level 2 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	18,478	17,658
Fair Value Inputs Level 2 [Member] | Mortgage and other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	384	343
Fair Value Inputs Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	495	629
Equity securities	33	103
Subtotal	528	732
Short-term investments	0	0
Total financial assets at fair value, excluding separate accounts	1,212	1,509
Financial liabilities at fair value:
Total financial liabilities at fair value	653	762
Fair Value Inputs Level 3 [Member] | Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	0	0
Financial liabilities at fair value:
Derivative liabilities	0	0
Fair Value Inputs Level 3 [Member] | Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	684	777
Financial liabilities at fair value:
Derivative liabilities	653	762
Fair Value Inputs Level 3 [Member] | Federal government and agency [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 3 [Member] | State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 3 [Member] | Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	44	45
Fair Value Inputs Level 3 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	307	430
Fair Value Inputs Level 3 [Member] | Mortgage and other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	$ 144	154
Fair Value Inputs Level 3 [Member] | Private equity securities [Member]
Financial assets at fair value:
Equity securities		$ 70