Fair Value Measurements - Financial Assets and Financial Liabilities Carried at Fair Value (Details) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2016 USD ($) Reinsurers	Dec. 31, 2015 USD ($) Reinsurers
Financial assets at fair value:
Fixed maturities		$ 21,244	$ 19,455
Equity securities		188	190
Short-term investments		997	381
Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	[1]	21,244	19,455
Equity securities		188	190
Subtotal		21,432	19,645
Short-term investments		997	381
Total financial assets at fair value, excluding separate accounts		23,367	20,949
Financial liabilities at fair value:
Total financial liabilities at fair value		912	885
Run-off Settlement Annuity Business [Member] | Fair Value, Measurements, Recurring [Member]
Financial liabilities at fair value:
Net appreciation required to adjust future policy benefits for run-off settlement annuity business included in fixed maturities		750	483
Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets		7	$ 16
Financial liabilities at fair value:
Derivative liabilities		$ 4
Guaranteed Minimum Income Benefit [Member]
Financial liabilities at fair value:
Number of external reinsurers | Reinsurers		3	3
Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	[2]	$ 931	$ 907
Financial liabilities at fair value:
Derivative liabilities		908	885
Federal government and agency [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		750	779
State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		1,508	1,641
Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		2,322	2,014
Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		16,165	14,448
Mortgage-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		36	49
Other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		463	524
Fair Value Inputs Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	[1]	180	251
Equity securities		4	32
Subtotal		184	283
Short-term investments		0	0
Total financial assets at fair value, excluding separate accounts		184	283
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
Fair Value Inputs Level 1 [Member] | Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	[2]	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		180	251
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-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		0	0
Fair Value Inputs Level 1 [Member] | 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	[1]	20,405	18,547
Equity securities		110	89
Subtotal		20,515	18,636
Short-term investments		997	381
Total financial assets at fair value, excluding separate accounts		21,519	19,033
Financial liabilities at fair value:
Total financial liabilities at fair value		4	0
Fair Value Inputs Level 2 [Member] | Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets		7	16
Financial liabilities at fair value:
Derivative liabilities		4
Fair Value Inputs Level 2 [Member] | Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	[2]	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		570	528
Fair Value Inputs Level 2 [Member] | State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		1,508	1,641
Fair Value Inputs Level 2 [Member] | Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		2,278	2,010
Fair Value Inputs Level 2 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		15,714	14,122
Fair Value Inputs Level 2 [Member] | Mortgage-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		36	48
Fair Value Inputs Level 2 [Member] | Other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		299	198
Fair Value Inputs Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	[1]	659	657
Equity securities		74	69
Subtotal		733	726
Short-term investments		0	0
Total financial assets at fair value, excluding separate accounts		1,664	1,633
Financial liabilities at fair value:
Total financial liabilities at fair value		908	885
Fair Value Inputs Level 3 [Member] | Run-off Settlement Annuity Business [Member] | Fair Value, Measurements, Recurring [Member]
Financial liabilities at fair value:
Net appreciation required to adjust future policy benefits for run-off settlement annuity business included in fixed maturities		19	30
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
Fair Value Inputs Level 3 [Member] | Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	[2]	931	907
Financial liabilities at fair value:
Derivative liabilities		908	885
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	4
Fair Value Inputs Level 3 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		451	326
Fair Value Inputs Level 3 [Member] | Mortgage-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		0	1
Fair Value Inputs Level 3 [Member] | Other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		$ 164	$ 326

[1] Fixed maturities included $750 million as of September 30, 2016 and $483 million as of December 31, 2015 of net appreciation required to adjust future policy benefits for the run-off settlement annuity business including $19 million as of September 30, 2016 and $30 million as of December 31, 2015 of appreciation for securities classified in Level 3. See Note 8 for additional information.

[2] The GMIB assets represent retrocessional contracts in place from three external reinsurers that cover the exposures on these contracts.