Interest rate swap agreements Outstanding interest rate swap (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) contract
Interest rate hedge agreements
Fair Values	$ (3,718)
Effective Portion of Interest Rate Cash Flow Hedge at Fiscal Year End	1,050,000
Notional Amount in Effect as of 2016	1,100,000
Notional Amount in Effect as of 2017	$ 650,000
0.53% Interest rate swap, effective December 31, 2014
Interest rate hedge agreements
Number of Interest Rate Derivatives Held | contract	3
Interest Pay Rate (as a percent)	0.53%
Fair Values	$ (94)
Effective Portion of Interest Rate Cash Flow Hedge at Fiscal Year End	500,000
Notional Amount in Effect as of 2016	0
Notional Amount in Effect as of 2017	$ 0
0.42% Interest rate swap, effective March 31, 2015
Interest rate hedge agreements
Number of Interest Rate Derivatives Held | contract	7
Interest Pay Rate (as a percent)	0.42%
Fair Values	$ 33
Effective Portion of Interest Rate Cash Flow Hedge at Fiscal Year End	450,000
Notional Amount in Effect as of 2016	0
Notional Amount in Effect as of 2017	$ 0
0.57% Interest rate swap, effective September 1, 2015
Interest rate hedge agreements
Number of Interest Rate Derivatives Held | contract	2
Interest Pay Rate (as a percent)	0.57%
Fair Values	$ 239
Effective Portion of Interest Rate Cash Flow Hedge at Fiscal Year End	100,000
Notional Amount in Effect as of 2016	100,000
Notional Amount in Effect as of 2017	$ 0
1.15% Interest rate swap, effective March 31, 2016
Interest rate hedge agreements
Number of Interest Rate Derivatives Held | contract	11
Interest Pay Rate (as a percent)	1.15%
Fair Values	$ (3,083)
Effective Portion of Interest Rate Cash Flow Hedge at Fiscal Year End	0
Notional Amount in Effect as of 2016	1,000,000
Notional Amount in Effect as of 2017	$ 0
1.51% Interest rate swap, effective March 31, 2017
Interest rate hedge agreements
Number of Interest Rate Derivatives Held | contract	11
Interest Pay Rate (as a percent)	1.51%
Fair Values	$ (813)
Effective Portion of Interest Rate Cash Flow Hedge at Fiscal Year End	0
Notional Amount in Effect as of 2016	0
Notional Amount in Effect as of 2017	$ 650,000
Unsecured Bank Term Loans [Member] | Minimum
Interest rate hedge agreements
Debt Instrument, Basis Spread on Variable Rate	1.20%
Unsecured Senior Line of Credit | Minimum
Interest rate hedge agreements
Debt Instrument, Basis Spread on Variable Rate	1.10%