Income taxes (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 28, 2016	Mar. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Minimum distribution of REIT taxable income, percent					90.00%
Taxable net income attributable to parent							$ 107,600
Dividends declared per common share (usd per share)	$ 0.15	$ 0.08	$ 0.15	$ 0.55	$ 0.45	$ 1.18	$ 1.83
Dividends on common stock					$ 24,648	$ 67,507	$ 103,900
Dividends on common stock		$ 4,600			$ 12,522	$ 0