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 Placement [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Private Placement [Member]
Balance at beginning of period	$ 28	$ 28
Gains (Losses) Realized	0	0
Gains (Losses) Unrealized	1	1
Purchases	9	10
Sales	(12)	(11)
Balance at end of period	$ 26	$ 28