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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income available for Common Stockholders – basic	$ 48,525	$ 40,998	$ 144,911	$ 127,071
Amounts allocated to dilutive securities	3,286	3,462	9,825	10,770
Net income available for Common Stockholders – fully diluted	$ 51,811	$ 44,460	$ 154,736	$ 137,841
Denominator:
Weighted average Common Shares outstanding – basic (shares)	87,037	85,105	86,620	84,649
Effect of dilutive securities:
Conversion of Common OP Units to Common Shares (shares)	5,836	7,203	6,100	7,205
Stock options and restricted shares (shares)	451	602	415	551
Weighted average Common Shares outstanding – fully diluted (shares)	93,324	92,910	93,135	92,405
Earnings per Common Share – Basic:
Net income available for Common Shares (usd per share)	$ 0.56	$ 0.48	$ 1.67	$ 1.50
Earnings per Common Share – Fully Diluted:
Net income available for Common Shares (usd per share)	$ 0.56	$ 0.48	$ 1.66	$ 1.49