Employee Benefit Plans (Reconciliations of Plan assets and the Projected Benefit Obligation, the Weighted-Average Assumptions Used to Determine Year-End Benefit Obligations, and the Amounts Recognized in Accumulated Other Comprehensive Income (Loss) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Fair value of plan assets at December 31	$ 2,531,000,000		$ 2,028,000,000
Qualified Plan
Fair value of plan assets at January 1	2,035,000,000		1,955,000,000
Actual return on plan assets	278,000,000		136,000,000		199,000,000
Employer contributions	350,000,000		0
Benefits paid	(122,000,000)	[1]	(56,000,000)
Fair value of plan assets at December 31	2,541,000,000		2,035,000,000		1,955,000,000
Projected benefit obligation at January 1	1,731,000,000		1,897,000,000
Service cost	29,000,000		37,000,000		33,000,000
Interest cost	88,000,000		80,000,000		79,000,000
Actuarial (gain) loss	344,000,000		(260,000,000)
Transfer between plans	0		33,000,000
Projected benefit obligation at December 31	2,070,000,000		1,731,000,000		1,897,000,000
Accumulated benefit obligation	1,905,000,000		1,598,000,000
Funded Status at December 31	471,000,000	[2],[3]	304,000,000	[2],[3]
Weighted-average assumptions, discount rate, percent	4.28%		5.17%
Weighted-average assumptions, rate of compensation increase, percent	3.75%		4.00%
Accumulated other comprehensive income (loss), net actuarial loss	(568,000,000)		(403,000,000)
Accumulated other comprehensive income (loss), net prior service (cost) credit	(25,000,000)		(31,000,000)
Balance at December 31	(593,000,000)		(434,000,000)
Lump sum settlements during fourth quarter of 2014, included in benefits paid	63,000,000
Non-Qualified Plan
Fair value of plan assets at January 1	0
Benefits paid	(10,000,000)		(9,000,000)
Fair value of plan assets at December 31	0		0
Projected benefit obligation at January 1	195,000,000		245,000,000
Service cost	3,000,000		4,000,000		4,000,000
Interest cost	10,000,000		9,000,000		10,000,000
Actuarial (gain) loss	37,000,000		(21,000,000)
Transfer between plans	0		(33,000,000)
Projected benefit obligation at December 31	235,000,000		195,000,000		245,000,000
Accumulated benefit obligation	203,000,000		163,000,000
Funded Status at December 31	(235,000,000)	[2],[3]	(195,000,000)	[2],[3]
Weighted-average assumptions, discount rate, percent	4.28%		5.17%
Weighted-average assumptions, rate of compensation increase, percent	3.75%		4.00%
Accumulated other comprehensive income (loss), net actuarial loss	(104,000,000)		(73,000,000)
Accumulated other comprehensive income (loss), net prior service (cost) credit	25,000,000		28,000,000
Balance at December 31	(79,000,000)		(45,000,000)
Postretirement Benefit Plan
Fair value of plan assets at January 1	67,000,000		72,000,000
Actual return on plan assets	3,000,000		(2,000,000)		4,000,000
Employer contributions	2,000,000		3,000,000
Benefits paid	(5,000,000)		(6,000,000)
Fair value of plan assets at December 31	67,000,000		67,000,000		72,000,000
Projected benefit obligation at January 1	69,000,000		79,000,000
Service cost	0		0
Interest cost	3,000,000		3,000,000		3,000,000
Actuarial (gain) loss	6,000,000		(7,000,000)
Transfer between plans	0		0
Projected benefit obligation at December 31	73,000,000		69,000,000		79,000,000
Accumulated benefit obligation	73,000,000		69,000,000
Funded Status at December 31	(6,000,000)	[2],[3]	(2,000,000)	[2],[3]
Weighted-average assumptions, discount rate, percent	3.99%		4.59%
Healthcare cost trend rate assumed for next year	7.00%		7.50%
Rate to which the healthcare cost trend rate is assumed to decline (the ultimate trend rate), percent	5.00%		5.00%		5.00%
Year when healthcare cost trend rate reaches the ultimate trend rate	2026		2033
Accumulated other comprehensive income (loss), net actuarial loss	(27,000,000)		(23,000,000)
Accumulated other comprehensive income (loss), net prior service (cost) credit	(3,000,000)		(3,000,000)
Balance at December 31	$ (30,000,000)		$ (26,000,000)

[1] Includes $63 million in benefit payments made to certain terminated vested eligible participants who elected to receive lump-sum settlements during the fourth quarter of 2014.

[2] Based on projected benefit obligation for defined benefit pension plans and accumulated benefit obligation for postretirement benefit plan.

[3] The Corporation recognizes the overfunded and underfunded status of the plans in “accrued income and other assets” and “accrued expenses and other liabilities,” respectively, on the consolidated balance sheets.