Equity and Capital Equity and Capital - Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Shares issued	183	18,125
Equity Issuances, Amount Available for Issuance	$ 70,800,000.0
Equity Offering, Common Shares available for issue	3,100
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
Maximum [Member]
Class of Stock [Line Items]
Equity Issuances, Amount Available for Issuance	200,000,000
AOCI Including Portion Attributable to Noncontrolling Interest [Member]
Class of Stock [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	2,453,000	2,249,000
Accumulated other comprehensive loss	$ (73,846,000)	(70,131,000)	$ (59,478,000)	$ (58,498,000)
Common Stock [Member]
Class of Stock [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 75.25
Equity Offering, Net Proceeds	$ 233,300,000
Accumulated Other Comprehensive Income (Loss) [Member]
Class of Stock [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	2,417,000	2,210,000
Accumulated other comprehensive loss	$ (72,893,000)	$ (69,277,000)	$ (58,693,000)	$ (57,748,000)