Subsequent events (Details) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended
	Jul. 31, 2016 USD ($) contract $ / shares	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)
Subsequent Event [Line Items]
Effective Portion of Interest Rate Cash Flow Hedge in Year Three		$ 400,000
Proceeds from the issuance of common stock		367,802	$ 5,052
Outstanding Balance		4,151,433
Forward Contracts [Member] | Subsequent Event
Subsequent Event [Line Items]
Sale of Stock, Price Per Share | $ / shares	$ 101.00
Proceeds from the issuance of common stock	$ 724,000
Unsecured Credit Facility [Member] | Subsequent Event
Subsequent Event [Line Items]
Extinguishment of Debt, Amount	2,400
2019 Unsecured Senior Bank Term Loan
Subsequent Event [Line Items]
Outstanding Balance		600,000
2019 Unsecured Senior Bank Term Loan | Subsequent Event
Subsequent Event [Line Items]
Extinguishment of Debt, Amount	869
Repayments of Debt	200,000
Outstanding Balance	$ 400,000
Interest Rate Hedge 0.95 Percent Transaction Date March 2018 [Member] | Subsequent Event
Subsequent Event [Line Items]
Number of Contracts | contract	2
Effective Portion of Interest Rate Cash Flow Hedge in Year Three	$ 200,000
Interest Pay Rate (as a percent)	0.95%
Line of Credit [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Borrowing Capacity, Description		$ 2,000,000
Maturity Date		Jan. 03, 2019
Outstanding Balance		$ 72,000
Line of Credit [Member] | Subsequent Event
Subsequent Event [Line Items]
Line of Credit Facility, Borrowing Capacity, Description	$ 1,650,000
Maturity Date	Oct. 29, 2021
London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member]
Subsequent Event [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR
Applicable margin (as a percent)		1.10%
London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member] | Subsequent Event
Subsequent Event [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Applicable margin (as a percent)	1.00%