Common Share Capitalization (Narrative) (Details) (USD $)	0 Months Ended		12 Months Ended			17 Months Ended	0 Months Ended
	Dec. 21, 2012	Aug. 04, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012	Aug. 20, 2012 Secondary Offering Public [Member]	Sep. 12, 2012 Secondary Offering Overallotment Option [Member]
Common Stock Capitalization [Line Items]
Common stock, shares issued			20,370,469	16,447,651		20,370,469	3,500,000	525,000
Net proceeds from issuance of shares							$ 76,700,000	$ 11,100,000
Overallotment option percentage by underwriters								15.00%
Dividends (in usd per share)			$ 2.50	$ 2.51	$ 2.45
Dividend Amount			47,400,000	42,400,000	33,900,000
Aggregate cost of shares repurchase		10,000,000
Shares repurchased	156,639		156,639	60,980	0	217,619
Repurchase price per share (in usd per share)	$ 21.90
Aggregate cost of shares repurchased	$ 3,430,000		$ 3,430,000	$ 1,051,000	$ 0	$ 4,500,000
Average price per share repurchased (in usd per share)						$ 20.59