Benefit Plans (Summary Of Restricted Stock Activity) (Details) (Performance Contingent Restricted Stock [Member], USD $)	3 Months Ended					12 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Mar. 31, 2013 Performance Criteria Not Met [Member]	Dec. 31, 2012 Performance Criteria Not Met [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	211,651		301,738
Nonvested shares, January 1, weighted average fair value (in dollars per share)	$ 32.42		$ 32.60
Granted (in shares)	49,093		45,612
Granted, weighted average fair value (In dollars per share)	$ 47.58		$ 43.05
Vested (in shares)	0	[1]	(65,172)	[1]
Vested, weighted average fair value (in dollars per share)	$ 0.00	[1]	$ 32.56	[1]
Forfeited (in shares)	(101,822)	[2]	(70,527)	[2]	(101,822)	(65,516)
Forfeited, weighted average fair value (in dollars per share)	$ 23.67	[2]	$ 39.93	[2]
Nonvested, March 31 (in shares/awards)	158,922		211,651
Nonvested shares, March 31, weighted average fair value (in dollars per share)	$ 42.71		$ 32.42

[1] In 2012, 65,172 performance-contingent restricted shares granted in 2010 vested because the specified performance criteria for such shares were met.

[2] In 2013 and 2012, 101,822 and 65,516 performance-contingent restricted shares granted in 2009 and 2008, respectively, were forfeited because the specified performance criteria for such shares were not met. The remaining forfeitures during 2012 were primarily caused by retirements and terminations of participants.