Noncontrolling interest (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Noncontrolling Interest [Line Items]
Balance, beginning of period	$ 1,566,260	$ 986,227	$ 1,266,376	$ 910,065
Issuance of shares	171,849	258,900	552,924	462,300
Income attributable to noncontrolling interest	21,193	22,561	58,724	40,739
Redemption of shares		(59,395)		(59,395)
Redemptions payable	(6,484)	(6,500)	(6,484)	(6,500)
Distributions	(8,381)	(12,759)	(127,103)	(158,175)
Balance, end of period	1,744,437	1,189,034	1,744,437	1,189,034
Redeemable noncontrolling interest
Noncontrolling Interest [Line Items]
Balance, beginning of period	1,409,037	834,644	1,111,714	617,791
Issuance of shares	112,500	249,300	381,250	452,700
Income attributable to noncontrolling interest	17,230	17,462	45,803	30,915
Redemption of shares		(59,395)		(59,395)
Redemptions payable	(6,484)	(6,500)	(6,484)	(6,500)
Distributions	0	0	0	0
Balance, end of period	1,532,283	1,035,511	1,532,283	1,035,511
Noncontrolling interest
Noncontrolling Interest [Line Items]
Balance, beginning of period	157,223	151,583	154,662	292,274
Issuance of shares	59,349	9,600	171,674	9,600
Income attributable to noncontrolling interest	3,963	5,099	12,921	9,824
Redemption of shares		0		0
Redemptions payable	0	0	0	0
Distributions	(8,381)	(12,759)	(127,103)	(158,175)
Balance, end of period	$ 212,154	$ 153,523	$ 212,154	$ 153,523