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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income available for Common Stockholders – basic	$ 56,070	$ 48,525	$ 162,429	$ 144,911
Amounts allocated to dilutive securities	3,590	3,286	10,569	9,825
Net income available for Common Stockholders – fully dilutive	$ 59,660	$ 51,811	$ 172,998	$ 154,736
Denominator:
Weighted average Common Shares outstanding – basic (shares)	89,200	87,037	88,760	86,620
Effect of dilutive securities:
Conversion of Common OP Units to Common Shares (shares)	5,771	5,836	5,808	6,100
Stock options and restricted shares (shares)	292	451	259	415
Weighted average Common Shares outstanding – fully diluted (shares)	95,263	93,324	94,827	93,135
Earnings per Common Share – Basic:
Net income available for Common Stockholders (usd per share)	$ 0.63	$ 0.56	$ 1.83	$ 1.67
Earnings per Common Share – Fully Diluted:
Net income available for Common Stockholders (usd per share)	$ 0.63	$ 0.56	$ 1.82	$ 1.66