Interest rate swap agreements Outstanding interest rate swap (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) contract	Jul. 31, 2016 USD ($) contract	Dec. 31, 2015 USD ($)
Interest rate hedge agreements
Fair Values	$ (12,222)
Notional Amount in Effect as of 3/31/16	1,100,000
Notional Amount in Effect as of 12/31/16	1,155,000
Notional Amount in Effect as of 12/31/17	1,026,000
Notional Amount in Effect as of 12/31/18	400,000
Interest rate hedge assets	$ 110		$ 596
Unsecured Bank Term Loan 2016 | Minimum
Interest rate hedge agreements
Applicable margin (as a percent)	1.20%
Unsecured Credit Facility [Member] | Minimum
Interest rate hedge agreements
Applicable margin (as a percent)	1.10%
Interest Rate Hedge .57 Percent Transaction Date September 2015 [Member]
Interest rate hedge agreements
Number of Contracts | contract	2
Interest Pay Rate (as a percent)	0.57%
Fair Values	$ (72)
Notional Amount in Effect as of 3/31/16	100,000
Notional Amount in Effect as of 12/31/16	100,000
Notional Amount in Effect as of 12/31/17	0
Notional Amount in Effect as of 12/31/18	$ 0
Interest Rate Hedge 1.15 Percent Transaction Date March 2016 [Member]
Interest rate hedge agreements
Number of Contracts | contract	11
Interest Pay Rate (as a percent)	1.15%
Fair Values	$ (5,058)
Notional Amount in Effect as of 3/31/16	1,000,000
Notional Amount in Effect as of 12/31/16	1,000,000
Notional Amount in Effect as of 12/31/17	0
Notional Amount in Effect as of 12/31/18	$ 0
Interest Rate Hedge 1.31 Percent Transaction Date March 2017 [Member]
Interest rate hedge agreements
Number of Contracts | contract	15
Interest Pay Rate (as a percent)	1.31%
Fair Values	$ (6,484)
Notional Amount in Effect as of 3/31/16	0
Notional Amount in Effect as of 12/31/16	0
Notional Amount in Effect as of 12/31/17	900,000
Notional Amount in Effect as of 12/31/18	$ 0
Interest Rate Hedge 1.06 Percent Transaction Date March 2018 [Member]
Interest rate hedge agreements
Number of Contracts | contract	4
Interest Pay Rate (as a percent)	1.06%
Fair Values	$ (718)
Notional Amount in Effect as of 3/31/16	0
Notional Amount in Effect as of 12/31/16	0
Notional Amount in Effect as of 12/31/17	0
Notional Amount in Effect as of 12/31/18	$ 250,000
Interest Rate Cap [Member]
Interest rate hedge agreements
Number of Contracts | contract	2
Interest Pay Rate (as a percent)	2.00%
Fair Values	$ 110
Notional Amount in Effect as of 3/31/16	0
Notional Amount in Effect as of 12/31/16	55,000
Notional Amount in Effect as of 12/31/17	126,000
Notional Amount in Effect as of 12/31/18	$ 150,000
Subsequent Event | Interest Rate Hedge 0.95 Percent Transaction Date March 2018 [Member]
Interest rate hedge agreements
Number of Contracts | contract		2
Interest Pay Rate (as a percent)		0.95%
Notional Amount in Effect as of 3/31/16		$ 0
Notional Amount in Effect as of 12/31/16		0
Notional Amount in Effect as of 12/31/17		0
Notional Amount in Effect as of 12/31/18		$ 200,000