Earnings Per Common Share - Calculation of numerator and denominator in eps table (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerators:
Net income available for Common Stockholders – basic	$ 35,490	$ 31,786	$ 86,073	$ 58,972
Amounts allocated to dilutive securities	2,998	2,724	7,308	5,054
Net income available for Common Stockholders – fully diluted	$ 38,488	$ 34,510	$ 93,381	$ 64,026
Denominator:
Weighted average Common Shares outstanding – basic (shares)	84,516	84,031	84,419	83,996
Effect of dilutive securities:
Redemption of Common OP Units for Common Shares (shares)	7,205	7,221	7,206	7,223
Stock options and restricted shares (shares)	543	599	538	610
Weighted average Common Shares outstanding – fully diluted (shares)	92,264	91,851	92,163	91,829
Earnings per Common Share – Basic:
Net income available for Common Shares (usd per share)	$ 0.42	$ 0.38	$ 1.02	$ 0.70
Earnings per Common Share – Fully Diluted:
Net income available for Common Shares (usd per share)	$ 0.42	$ 0.38	$ 1.01	$ 0.70