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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income available for Common Stockholders – basic	$ 46,137	$ 39,498	$ 106,359	$ 96,385
Amounts allocated to dilutive securities	3,024	2,649	6,979	6,539
Net income available for Common Stockholders – fully diluted	$ 49,161	$ 42,147	$ 113,338	$ 102,924
Denominator:
Weighted average Common Shares outstanding – basic (shares)	88,549	86,763	88,537	86,408
Effect of dilutive securities:
Conversion of Common OP Units to Common Shares (shares)	5,826	5,886	5,827	6,235
Stock options and restricted shares (shares)	248	414	236	398
Weighted average Common Shares outstanding – fully diluted (shares)	94,623	93,063	94,600	93,041
Earnings per Common Share – Basic:
Net income available for Common Stockholders (usd per share)	$ 0.52	$ 0.46	$ 1.20	$ 1.12
Earnings per Common Share – Fully Diluted:
Net income available for Common Stockholders (usd per share)	$ 0.52	$ 0.45	$ 1.20	$ 1.11