Equity and Capital Equity and Capital - Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jan. 15, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Document Fiscal Year Focus			2015
Shares issued		0	18
Equity Issuances, Amount Available for Issuance	$ 95,000,000		$ 95,000,000
Document Period End Date			Jun. 30, 2015
Equity Offering, Common Shares available for issue					2,875
Weighted Average Price Per Share			$ 64.72
Net proceeds from common stock issuance			$ 1,173,000
Payments for Commissions			15,000
Payments of Stock Issuance Costs			0
Equity Offering, Net Proceeds			193,800,000
Maximum [Member]
Class of Stock [Line Items]
Equity Issuances, Amount Available for Issuance	200,000,000		200,000,000
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	2,250,000	$ 2,318,000	4,500,000	$ 4,837,000
Accumulated Other Comprehensive Income (Loss) [Member]
Class of Stock [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			4,500,000	4,837,000
Accumulated other comprehensive loss	$ (49,534,000)	$ (37,085,000)	$ (49,534,000)	$ (37,085,000)		$ (58,498,000)	$ (17,883,000)