Share-Based Compensation Arrangements (Nonvested Share Units Activity) (Details) (Time Based Nonvested Shares [Member], USD $)	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Time Based Nonvested Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity Instruments Other than Options	109,975		91,250
Equity Instruments Other than Options Weighted Average Grant Date Fair Value	$ 20.70		$ 19.84
Equity Instruments Other than Options Granted	27,725	31,950
Equity Instruments Other than Options Granted Weighted Average Grant Date Fair Value	$ 23.54
Equity Instruments Other than Options Vested	(4,819)
Equity Instruments Other than Options Vested Weighted Average Grant Date Fair Value	$ 19.60
Equity Instruments Other than Options Forfeited	(4,181)
Equity Instruments Other than Options Forfeited Weighted Average Grant Date Fair Value	$ 21.94