Noncontrolling interests (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Noncontrolling Interest [Line Items]
Total, beginning of period				$ 1,266,376				$ 910,065	$ 1,266,376	$ 910,065
Issuance of shares									565,124	508,800
Income attributable to noncontrolling interest	$ 27,200	$ 37,439	$ 21,193	37,531	$ 25,996	$ 26,229	$ 22,561	18,178	123,363	92,964	$ 74,880
Redemption of shares / Distributions									(260,885)	(245,453)
Total, end of period	1,693,978				1,266,376				1,693,978	1,266,376	910,065
Redeemable Noncontrolling Interest
Noncontrolling Interest [Line Items]
Total, beginning of period				1,111,714				617,791	1,111,714	617,791
Issuance of shares									393,450	499,200
Income attributable to noncontrolling interest									100,852	71,418
Redemption of shares / Distributions									(78,015)	(76,695)
Total, end of period	1,528,001				1,111,714				1,528,001	1,111,714	617,791
Amounts payable to concontrolling interest	71,530				10,800				71,530	10,800
Noncontrolling Interest
Noncontrolling Interest [Line Items]
Total, beginning of period				$ 154,662				$ 292,274	154,662	292,274
Issuance of shares									171,674	9,600
Income attributable to noncontrolling interest									22,511	21,546
Redemption of shares / Distributions									(182,870)	(168,758)
Total, end of period	165,977				154,662				165,977	154,662	$ 292,274
Amounts payable to concontrolling interest	$ 16,144				$ 0				$ 16,144	$ 0