Equity Compensation Equity Compensation Deferred Directors (Details) (Director [Member], Director Deferred Share Units [Member], USD $)	12 Months Ended
	Dec. 31, 2012
Director [Member] | Director Deferred Share Units [Member]
Director deferred share units [Roll Forward]
Director deferred share units, beginning balance (in shares)	54,231
Director deferred share units, issued (in shares)	15,587
Director deferred share units, ending balance (in shares)	69,818
Weighted-average grant date fair value, director deferred share units, beginning balance (in dollars per share)	$ 20.19	[1]
Weighted-average grant date fair value, director deferred share units, issued (in dollars per share)	$ 20.96	[1]
Weighted-average grant date fair value, director deferred share units, ending balance (in dollars per share)	$ 20.36	[1]

[1] * weighted-average