11. Stockholders Equity of the REIT and Partners Capital of the OP Stockholders Equity of the REIT and Partners Capital of the OP Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	2 Months Ended	3 Months Ended												12 Months Ended
	Feb. 23, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stockholders’ Equity of the REIT and Partners’ Capital of the OP [Line Items]
Issuance of stock awards, shares														227,430	565,162	163,187
Redemption of operating partnership units, shares	1,409,147													1,618,048	363,674	234,300
Stock Repurchase Program, Authorized Amount						$ 120,000									$ 120,000
Common stock repurchase, shares															1,002,610
Stock Repurchased and Retired During Period, Value															$ 31,912
Stock Issued During Period, Shares, New Issues														7,613,000
Shares Issued, Price Per Share		$ 37.75												$ 37.75
Issuance of common stock, full exercise of underwriters' option														993,000
Stock Issued During Period, Value, New Issues														$ 275,470
Common Stock [Member]
Stockholders’ Equity of the REIT and Partners’ Capital of the OP [Line Items]
Cash Dividend		0.50	$ 0.47	$ 0.47	$ 0.47	$ 0.47	$ 0.42	$ 0.42	$ 0.42	$ 0.42	$ 0.35	$ 0.35	$ 0.35	$ 1.91	$ 1.73	$ 1.47
Ordinary Taxable Dividend		$ 0.00	$ 0.26	$ 0.26	$ 0.26	$ 0.33	$ 0.42	$ 0.42	$ 0.42	$ 0.390	$ 0.350	$ 0.350	$ 0.350	$ 0.78	$ 1.59	$ 1.440
Stock Issued During Period, Shares, New Issues														7,613,000
January Two Zero One Six [Member] | Common Stock [Member]
Stockholders’ Equity of the REIT and Partners’ Capital of the OP [Line Items]
Ordinary Taxable Dividend														$ 0.14
January Two Zero One Five [Member] | Common Stock [Member]
Stockholders’ Equity of the REIT and Partners’ Capital of the OP [Line Items]
Ordinary Taxable Dividend														$ 0.03