Equity and Capital Equity and Capital - Common Stock (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Jan. 15, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Document Fiscal Year Focus			2015
Shares issued	0	871,754,000	18,125	871,754,000
Equity Issuances, Amount Available for Issuance	$ 95,000,000		$ 95,000,000
Document Period End Date			Sep. 30, 2015
Equity Offering, Common Shares available for issue					2,875,000
Weighted Average Price Per Share		$ 57.35	$ 64.72	$ 57.35
Net proceeds from common stock issuance		$ 49,995,000	$ 1,173,000	$ 49,995,000
Payments for Commissions		695,000	15,000	695,000
Payments of Stock Issuance Costs		0	0	0
Maximum [Member]
Class of Stock [Line Items]
Equity Issuances, Amount Available for Issuance	200,000,000		200,000,000
Common Stock [Member]
Class of Stock [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 67.40
Equity Offering, Net Proceeds			193,800,000
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Class of Stock [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	2,155,000	2,260,000	6,654,000	7,098,000
Accumulated Other Comprehensive Income (Loss) [Member]
Class of Stock [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			6,654,000	7,098,000
Accumulated other comprehensive loss	$ (63,191,000)	$ (34,579,000)	$ (63,191,000)	$ (34,579,000)		$ (58,498,000)	$ (17,883,000)