Equity and Capital Equity and Capital - Common Stock (Details) - USD ($)		3 Months Ended		9 Months Ended
	Jul. 15, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jul. 13, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Document Period End Date				Sep. 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		122,805,000		122,805,000
Long-term Debt, Maturities, Repayments of Principal in Year Three		$ 62,184,000		62,184,000
Payments of Stock Issuance Costs				$ 80,000	$ 0
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		$ 43,111,000	$ 2,155,000	$ 48,063,000	6,654,000
Accumulated other comprehensive loss		$ (36,663,000)	(63,191,000)	$ (36,663,000)	(63,191,000)		$ (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				$ 47,880,000	6,536,000
Accumulated other comprehensive loss		$ (35,739,000)	$ (62,307,000)	$ (35,739,000)	$ (62,307,000)		$ (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]
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]
Class of Stock [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						5.875%