Equity and Capital Equity and Capital - Common Stock (Details) - USD ($)		3 Months Ended		6 Months Ended
	Jul. 15, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jul. 13, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Document Period End Date				Jun. 30, 2016
Equity Issuances, Amount Available for Issuance		$ 70,800,000.0		$ 70,800,000.0
Long-term Debt, Maturities, Repayments of Principal in Year Two		423,076,000		423,076,000
Long-term Debt, Maturities, Repayments of Principal in Year Three		$ 62,461,000		$ 62,461,000
Maximum [Member]
Class of Stock [Line Items]
Equity Issuances, Common Shares Authorized for Issuance		200,000,000		200,000,000
Equity Offering, Common Shares Authorized for Issuance		3,100,000		3,100,000
Forward Equity Offering, Common Shares available for issue		1,250,000		1,250,000
AOCI Including Portion Attributable to Noncontrolling Interest [Member]
Class of Stock [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		$ 2,500,000	$ 2,250,000	$ 4,952,000	$ 4,500,000
Accumulated other comprehensive loss		$ (81,120,000)	(49,534,000)	$ (81,120,000)	(49,534,000)		$ (59,478,000)	$ (58,498,000)
Common Stock [Member]
Class of Stock [Line Items]
Forward Equity Offering, Agreement Price Per Share, Common Stock		$ 75.25		$ 75.25
Accumulated Other Comprehensive Income (Loss) [Member]
Class of Stock [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				$ 4,874,000	4,421,000
Accumulated other comprehensive loss		$ (80,038,000)	$ (48,799,000)	$ (80,038,000)	$ (48,799,000)		$ (58,693,000)	$ (57,748,000)
ATM Equity Offering Program [Member]
Class of Stock [Line Items]
Shares issued				182,787	18,125
Weighted Average Price Per Share				$ 68.85	$ 64.72
Net proceeds from common stock issuance				$ 12,584,000	$ 1,173,000
Payments for Commissions				$ 157,000	$ 15,000
Forward Equity Offering [Member]
Class of Stock [Line Items]
Shares issued				1,850,000
Net proceeds from common stock issuance				$ 137,500,000
Equity Offering [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Shares issued	5,000,000
Shares Issued, Price Per Share	$ 79.78
Net proceeds from common stock issuance	$ 400,100,000
Unsecured Debt [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Notice of Repayment of Unsecured Debt, Subsequent to Period End Date						$ 300,000,000
Debt Instrument, Interest Rate, Stated Percentage						5.875%