Fair Value Measurements - Financial Assets and Financial Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Financial assets at fair value:
Fixed maturities	$ 24,178	$ 23,138
Equity securities	567	588
Short-term investments	245	199
Financial liabilities at fair value:
Unfunded Commitments	$ 67
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:
Equity securities	$ 501	588
Subtotal	24,679	23,726
Short-term investments	245	199
Total financial assets at fair value, excluding separate accounts	25,643	24,704
Real estate funds priced at NAV as a practical expedient	273
Financial liabilities at fair value:
Total financial liabilities at fair value	$ 726	787
Redemption Frequency	Quarterly redemption frequency
Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	$ 2	2
Financial liabilities at fair value:
Derivative liabilities	44	25
Guaranteed Minimum Income Benefit [Member]
Financial assets at fair value:
Derivative assets	717	777
Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	717	777
Financial liabilities at fair value:
Derivative liabilities	682	762
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	24,178	23,138
Federal government and agency [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	868	779
State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	1,129	1,287
Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	2,499	2,487
Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	19,173	18,088
Mortgage and other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	509	497
Fair Value Inputs Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Equity securities	399	412
Subtotal	755	665
Short-term investments	0	0
Total financial assets at fair value, excluding separate accounts	755	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] | Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	356	253
Fair Value Inputs Level 1 [Member] | Federal government and agency [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	356	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:
Equity securities	69	73
Subtotal	23,328	22,329
Short-term investments	245	199
Total financial assets at fair value, excluding separate accounts	23,575	22,530
Financial liabilities at fair value:
Total financial liabilities at fair value	44	25
Fair Value Inputs Level 2 [Member] | Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	2	2
Financial liabilities at fair value:
Derivative liabilities	44	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] | Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	23,259	22,256
Fair Value Inputs Level 2 [Member] | Federal government and agency [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	512	526
Fair Value Inputs Level 2 [Member] | State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	1,129	1,287
Fair Value Inputs Level 2 [Member] | Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	2,454	2,442
Fair Value Inputs Level 2 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	18,802	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	362	343
Fair Value Inputs Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Equity securities	33	103
Subtotal	596	732
Short-term investments	0	0
Total financial assets at fair value, excluding separate accounts	1,313	1,509
Financial liabilities at fair value:
Total financial liabilities at fair value	682	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	717	777
Financial liabilities at fair value:
Derivative liabilities	682	762
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	563	629
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	45	45
Fair Value Inputs Level 3 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	371	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	$ 147	154
Fair Value Inputs Level 3 [Member] | Private equity securities [Member]
Financial assets at fair value:
Equity securities		$ 70