Benefit Plans (Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Pension benefits
Pension plans' assets
Percentage of total plan assets for primary qualified employee				80.00%
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets				100.00%	100.00%
Notional amount of derivatives, minimum percentage of plan assets	105.00%
Notional amount of derivatives, maximum percentage of plan assets	115.00%
Total plan assets at fair value				$ 5,758	$ 5,501
Total plan assets at fair value, allocation (as a percent)	100.00%	100.00%
Securities lending obligation				(158)	(167)
Other net plan assets				50	19
Total reported plan assets	$ 5,353	$ 5,783	$ 5,783	$ 5,650	5,353
Pension plan assets
Fair value of plan assets, beginning of year	5,353	5,783
Fair value of plan assets, end of year	$ 5,650	$ 5,353	$ 5,783
Expected weighted average long-term rate of return on plan assets	7.30%	7.33%	7.36%
Expected weighted average long-term rate of return on plan assets for the next fiscal year (as a percent)	7.31%
Expected weighted average long-term rate of return on primary employee plan assets for the next fiscal year (as a percent)	7.75%
Expected weighted average long-term rate of return on employee-agent plan assets for the next fiscal year (as a percent)	5.75%
Arithmetic average of the annual actual return on plan assets for the last 10 years (as a percent)				6.50%
Arithmetic average of the annual actual return on plan assets for the last 5 years (as a percent)				8.50%
Cash flows
Defined benefit pension plans, estimated employer contributions in next fiscal year	$ 136
Employer contribution	131	$ 125
Participant contributions	$ 1	$ 0
Estimated future benefit payments
2017				$ 412
2018				421
2019				463
2020				487
2021				521
2022-2026				2,512
Total benefit payments				4,816
Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				360	342
Total plan assets at fair value, allocation (as a percent)	6.30%	6.20%
Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				4,946	4,668
Total plan assets at fair value, allocation (as a percent)	85.90%	84.90%
Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				452	491
Total plan assets at fair value, allocation (as a percent)	7.80%	8.90%
Total reported plan assets	$ 491	$ 505	$ 690	452	491
Pension plan assets
Fair value of plan assets, beginning of year	491	505	690
Actual return on plan assets: Relating to assets sold during the period	(2)	1	(5)
Actual return on plan assets: Relating to assets still held at the reporting date	26	(24)	12
Purchases, sales and settlements, net	(68)	11	(192)
Net transfers in and/or (out) of Level 3	5	(2)	0
Fair value of plan assets, end of year	452	491	505
Postretirement benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total reported plan assets	0	0		0	$ 0
Pension plan assets
Fair value of plan assets, beginning of year	0
Fair value of plan assets, end of year	0	0
Cash flows
Employer contribution	25	35
Participant contributions	$ 16	19
Estimated future benefit payments
2017				23
2018				23
2019				24
2020				25
2021				26
2022-2026				137
Total benefit payments				$ 258
Equity securities | Pension benefits
Pension plans' assets
Percent of total equity securities in primary qualified employee plan	86.00%
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation, minimum	50.00%
Target asset allocation, maximum	68.00%
Actual percentage of plan assets				62.00%	60.00%
Total plan assets at fair value				$ 3,464	$ 3,181
Equity securities | Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				155	136
Equity securities | Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				3,230	2,945
Equity securities | Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				79	100
Total reported plan assets	$ 100	75	237	$ 79	$ 100
Pension plan assets
Fair value of plan assets, beginning of year	100	75	237
Actual return on plan assets: Relating to assets sold during the period	(2)	1	2
Actual return on plan assets: Relating to assets still held at the reporting date	(1)	(5)	2
Purchases, sales and settlements, net	(18)	29	(166)
Net transfers in and/or (out) of Level 3	0	0	0
Fair value of plan assets, end of year	$ 79	100	75
Fixed income securities | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation, minimum	27.00%
Target asset allocation, maximum	37.00%
Actual percentage of plan assets				29.00%	30.00%
U.S. government and agencies | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				$ 315	$ 406
U.S. government and agencies | Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				30	72
U.S. government and agencies | Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				285	334
U.S. government and agencies | Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0	0
Municipal | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value					7
Municipal | Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value					0
Municipal | Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value					0
Municipal | Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value					7
Total reported plan assets	$ 7	14	18	0	7
Pension plan assets
Fair value of plan assets, beginning of year	7	14	18
Actual return on plan assets: Relating to assets sold during the period	0	0	0
Actual return on plan assets: Relating to assets still held at the reporting date	0	0	0
Purchases, sales and settlements, net	(7)	(7)	(4)
Net transfers in and/or (out) of Level 3	0	0	0
Fair value of plan assets, end of year	0	7	14
Corporate | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				1,319	1,215
Corporate | Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0	0
Corporate | Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				1,309	1,205
Corporate | Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				10	10
Total reported plan assets	10	12	18	10	10
Pension plan assets
Fair value of plan assets, beginning of year	10	12	18
Actual return on plan assets: Relating to assets sold during the period	0	0	0
Actual return on plan assets: Relating to assets still held at the reporting date	0	0	0
Purchases, sales and settlements, net	(5)	0	(6)
Net transfers in and/or (out) of Level 3	5	(2)	0
Fair value of plan assets, end of year	$ 10	10	12
Securities pledged as collateral | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Assets for plan benefits				$ 143	$ 152
Limited partnership interests | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation, minimum	0.00%
Target asset allocation, maximum	14.00%
Actual percentage of plan assets				7.00%	7.00%
Short-term investments | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				$ 265	$ 296
Short-term investments | Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				144	112
Short-term investments | Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				121	184
Short-term investments | Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				$ 0	$ 0
Short-term investments and other | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	0.00%
Actual percentage of plan assets				2.00%	3.00%
Real estate funds | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				$ 100	$ 104
Real estate funds | Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0	0
Real estate funds | Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0	0
Real estate funds | Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				100	104
Total reported plan assets	$ 104	154	197	100	104
Pension plan assets
Fair value of plan assets, beginning of year	104	154	197
Actual return on plan assets: Relating to assets sold during the period	0	0	(3)
Actual return on plan assets: Relating to assets still held at the reporting date	5	(12)	6
Purchases, sales and settlements, net	(9)	(38)	(46)
Net transfers in and/or (out) of Level 3	0	0	0
Fair value of plan assets, end of year	100	104	154
Private equity funds | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				261	237
Private equity funds | Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0	0
Private equity funds | Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0	0
Private equity funds | Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				261	237
Total reported plan assets	237	218	211	261	237
Pension plan assets
Fair value of plan assets, beginning of year	237	218	211
Actual return on plan assets: Relating to assets sold during the period	0	0	(4)
Actual return on plan assets: Relating to assets still held at the reporting date	24	(8)	4
Purchases, sales and settlements, net	0	27	7
Net transfers in and/or (out) of Level 3	0	0	0
Fair value of plan assets, end of year	261	237	218
Hedge funds | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				2	33
Hedge funds | Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0	0
Hedge funds | Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0	0
Hedge funds | Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				2	33
Total reported plan assets	33	32	9	2	33
Pension plan assets
Fair value of plan assets, beginning of year	33	32	9
Actual return on plan assets: Relating to assets sold during the period	0	0	0
Actual return on plan assets: Relating to assets still held at the reporting date	(2)	1	0
Purchases, sales and settlements, net	(29)	0	23
Net transfers in and/or (out) of Level 3	0	0	0
Fair value of plan assets, end of year	$ 2	$ 33	$ 32
Cash and cash equivalents | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				32	22
Cash and cash equivalents | Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				32	22
Cash and cash equivalents | Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0	0
Cash and cash equivalents | Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0	$ 0
Free Standing Derivative Assets | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				0
Free Standing Derivative Assets | Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				(1)
Free Standing Derivative Assets | Pension benefits | Significant other observable inputs (Level 2)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				1
Free Standing Derivative Assets | Pension benefits | Significant unobservable inputs (Level 3)
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Total plan assets at fair value				$ 0
Equity securities | Fixed income securities | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets				3.00%	3.00%
Equity securities | Derivatives | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets				1.00%
Equity securities | Private equity funds | Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets				1.00%	2.00%