Stockholders' equity (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 18, 2014	Apr. 30, 2008	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Apr. 30, 2012
Issuances of common stock
Common stock, shares authorized under the plan			100,000,000	100,000,000	100,000,000
Proceeds from common stock offerings				$ 78,463	$ 0	$ 534,469
Preferred stock redemption charge				$ 0	1,989	$ 0
Stock Redeemed or Called During Period, Value					$ 14,414
Preferred stock, shares authorized			100,000,000	100,000,000
Number of shares issued and outstanding			14,700,000	14,700,000
Number of "excess stock" authorized (in shares)				200,000,000
Number of excess stock authorized issued and outstanding (in shares)			0	0
Sales of noncontrolling interests				$ 443,445
Series D Convertible Preferred Stock
Issuances of common stock
Series D Preferred Stock, Shares Outstanding			9,486,500	9,486,500	10,000,000
Payment of quarterly dividends in arrears at an annual rate (in dollars per share)		$ 1.75
Redemption price per share	$ 27.975		$ 25.00	$ 25.00			$ 25.00
Number of share issued and outstanding	513,500
Stock Redeemed or Called During Period, Value	$ 14,400
Percentage of the closing sale price per share of the entity's common stock that the then-applicable conversion price must exceed in order for the shares to be automatically converted		150.00%
Minimum number of trading days within 30 consecutive trading days during which the closing sale price per share of entity's common stock equals or exceeds the then-applicable conversion price for the shares to be automatically converted		20 days
Number of consecutive trading day period within which the closing sale price per share of entity's common stock equals or exceeds the then-applicable conversion price for at least 20 trading days for the shares to be automatically converted		30 days
Conversion rate at a option of holders (in shares)		0.2477
Conversion rate which is equivalent to an initial conversion price per share of common stock (in dollars per share)		$ 100.93
Conversion rate dividend adjustment per quarter (in dollars per share)		$ 0.78
Conversion rate (in shares)			0.2480	0.2480
Conversion price (in dollars per share)			$ 100.81	$ 100.81
Preferred stock, shares authorized			10,000,000	10,000,000	10,000,000
Number of shares issued and outstanding			9,486,500	9,486,500	10,000,000
Write-off of issuance costs | Series D Convertible Preferred Stock
Issuances of common stock
Preferred stock redemption charge				$ (412)
Common Stock
Issuances of common stock
Issuance of common stock (in shares)				889,856	291,679	7,590,000
Additional Paid-In Capital
Issuances of common stock
Stock Redeemed or Called During Period, Value					$ (412)
Sales of noncontrolling interests				$ 141,850
At the Market Common Stock Offering Program, Established December 2015
Issuances of common stock
Common stock, shares authorized under the plan			450,000,000	450,000,000
Issuance of common stock (in shares)				832,982
Gross proceeds from shares sold			$ 75,000
Common stock available for future issuance			$ 375,000	$ 375,000
At the Market Equity Offering Programs
Issuances of common stock
Issuance of common stock (in shares)				889,856
Gross proceeds from shares sold				$ 80,300
Average stock price (per share)			$ 90.29	$ 90.29
Proceeds from common stock offerings				$ 78,500
Commissions and other expenses				$ 1,800
At the Market Common Stock Offering Program, Expired June 2015
Issuances of common stock
Issuance of common stock (in shares)				56,874