Debt (Revolving Credit Facilities) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Line of Credit Facility [Line Items]
Maximum balance outstanding during the year	$ 281,700	$ 330,700
Weighted average balance	169,746	151,814
Year-to-date weighted average interest rate (excluding facility fee)	1.30%	1.50%
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	170,000	145,000
Available balance	326,571	351,571
Letter of credit outstanding under facility	3,429	3,429
Variable interest rate	1.20%	1.30%
Unsecured And Uncommitted Overnight Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 59,000	$ 21,500
Variable interest rate	1.50%	1.50%