Fair Value Measurements - Financial Assets and Financial Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financial assets at fair value:
Fixed maturities	$ 23,138	$ 20,961
Equity securities	588	583
Short-term investments	199	691
Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	23,138	20,961
Equity securities	588	583
Subtotal	23,726	21,544
Short-term investments	199	691
Total financial assets at fair value, excluding separate accounts	24,704	23,044
Financial liabilities at fair value:
Total financial liabilities at fair value	787	785
Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	2	10
Financial liabilities at fair value:
Derivative liabilities	25	5
Guaranteed Minimum Income Benefit [Member]
Financial assets at fair value:
Derivative assets	777	799
Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	777	799
Financial liabilities at fair value:
Derivative liabilities	762	780
Federal government and agency [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	779	877
State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	1,287	1,435
Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	2,487	2,113
Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	18,088	16,050
Mortgage and other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	497	486
Fair Value Inputs Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	253	374
Equity securities	412	396
Subtotal	665	770
Short-term investments	0	0
Total financial assets at fair value, excluding separate accounts	665	770
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	253	374
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	22,256	19,885
Equity securities	73	113
Subtotal	22,329	19,998
Short-term investments	199	691
Total financial assets at fair value, excluding separate accounts	22,530	20,699
Financial liabilities at fair value:
Total financial liabilities at fair value	25	5
Fair Value Inputs Level 2 [Member] | Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	2	10
Financial liabilities at fair value:
Derivative liabilities	25	5
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	526	503
Fair Value Inputs Level 2 [Member] | State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	1,287	1,435
Fair Value Inputs Level 2 [Member] | Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	2,442	2,066
Fair Value Inputs Level 2 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	17,658	15,552
Fair Value Inputs Level 2 [Member] | Mortgage and other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	343	329
Fair Value Inputs Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	629	702
Equity securities	103	74
Subtotal	732	776
Short-term investments	0	0
Total financial assets at fair value, excluding separate accounts	1,509	1,575
Financial liabilities at fair value:
Total financial liabilities at fair value	762	780
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	777	799
Financial liabilities at fair value:
Derivative liabilities	762	780
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	47
Fair Value Inputs Level 3 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	430	498
Fair Value Inputs Level 3 [Member] | Mortgage and other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	$ 154	$ 157