Fair Value Measurements - Financial Assets and Financial Liabilities Carried at Fair Value (Details) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Financial assets at fair value:
Fixed maturities		$ 20,087	$ 19,455
Equity securities		194	190
Short-term investments		465	381
Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	[1]	20,087	19,455
Equity securities		194	190
Subtotal		20,281	19,645
Short-term investments		465	381
Total financial assets at fair value, excluding separate accounts		21,714	20,949
Financial liabilities at fair value:
Total financial liabilities at fair value		942	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		622	483
Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	[2]	11	16
Financial liabilities at fair value:
Derivative liabilities		1
Other derivatives [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets		10	15
Other derivatives [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets		$ 1	$ 1
Guaranteed Minimum Income Benefit [Member]
Financial liabilities at fair value:
Number of external reinsurers		3	3
Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	[3]	$ 957	$ 907
Financial liabilities at fair value:
Derivative liabilities		941	885
Federal government and agency [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		752	779
State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		1,603	1,641
Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		2,072	2,014
Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		15,067	14,448
Mortgage-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		41	49
Other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		552	524
Fair Value Inputs Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	[1]	183	251
Equity securities		33	32
Subtotal		216	283
Short-term investments		0	0
Total financial assets at fair value, excluding separate accounts		216	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	[2]	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	[3]	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		183	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]	19,184	18,547
Equity securities		89	89
Subtotal		19,273	18,636
Short-term investments		465	381
Total financial assets at fair value, excluding separate accounts		19,749	19,033
Financial liabilities at fair value:
Total financial liabilities at fair value		1	0
Fair Value Inputs Level 2 [Member] | Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	[2]	11	16
Financial liabilities at fair value:
Derivative liabilities		1
Fair Value Inputs Level 2 [Member] | Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	[3]	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		569	528
Fair Value Inputs Level 2 [Member] | State and local government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		1,603	1,641
Fair Value Inputs Level 2 [Member] | Foreign government [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		2,068	2,010
Fair Value Inputs Level 2 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		14,661	14,122
Fair Value Inputs Level 2 [Member] | Mortgage-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		40	48
Fair Value Inputs Level 2 [Member] | Other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		243	198
Fair Value Inputs Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	[1]	720	657
Equity securities		72	69
Subtotal		792	726
Short-term investments		0	0
Total financial assets at fair value, excluding separate accounts		1,749	1,633
Financial liabilities at fair value:
Total financial liabilities at fair value		941	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		42	30
Fair Value Inputs Level 3 [Member] | Other derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	[2]	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	[3]	957	907
Financial liabilities at fair value:
Derivative liabilities		941	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		4	4
Fair Value Inputs Level 3 [Member] | Corporate [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		406	326
Fair Value Inputs Level 3 [Member] | Mortgage-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		1	1
Fair Value Inputs Level 3 [Member] | Other asset-backed [Member] | Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities		$ 309	$ 326

[1] Fixed maturities included $622 million as of March 31, 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 $42 million as of March 31, 2016 and $30 million as of December 31, 2015 of appreciation for securities classified in Level 3. See Note 8 for additional information.

[2] Other derivative assets included $10 million as of March 31, 2016 and $15 million as of December 31, 2015 of interest rate and foreign currency swaps qualifying as cash flow hedges and $1 million as of March 31, 2016 and December 31, 2015 of interest rate swaps qualifying as fair value hedges. See Note 9 for additional information.

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