Income taxes (Details) (USD $)	0 Months Ended			3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 30, 2015	Mar. 11, 2014	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Minimum distribution of REIT taxable income, amount						$ 55,300,000
Dividends declared per common share	$ 0.55	$ 0.08	$ 0.08	$ 0.63	$ 0.48	$ 2.03
Dividends on common stock		4,600,000	4,500,000	4,576,000	27,398,000	116,000,000
Minimum distribution of REIT taxable income, percent				90.00%
Taxable net income (loss) attributable to parent						115,800,000
Net capital gains						$ 54,400,000