Employee Benefit Plans (Summary of Changes in the Corporation's Qualified Defined Benefit Pension Plan's Level 3 Investments Measured at Fair Value on a Recurring Basis) (Details) (Private placements, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Private placements
Balance at beginning of period	$ 36	$ 30
Gains (Losses) Realized	1	0
Gains (Losses) Unrealized	4	(4)
Purchases	60	46
Sales	(28)	(36)
Balance at end of period	$ 73	$ 36