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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerators:
Net income available for Common Stockholders – basic	$ 40,998	$ 36,673	$ 127,071	$ 95,644
Amounts allocated to dilutive securities	3,462	3,136	10,770	8,191
Net income available for Common Stockholders – fully diluted	$ 44,460	$ 39,809	$ 137,841	$ 103,835
Denominator:
Weighted average Common Shares outstanding – basic (shares)	85,105	84,057	84,649	84,016
Effect of dilutive securities:
Redemption of Common OP Units for Common Shares (shares)	7,203	7,212	7,205	7,220
Stock options and restricted shares (shares)	602	671	551	641
Weighted average Common Shares outstanding – fully diluted (shares)	92,910	91,940	92,405	91,877
Earnings per Common Share – Basic:
Net income available for Common Shares (usd per share)	$ 0.48	$ 0.44	$ 1.50	$ 1.14
Earnings per Common Share – Fully Diluted:
Net income available for Common Shares (usd per share)	$ 0.48	$ 0.43	$ 1.49	$ 1.13