Earnings per share - Summary of preferred stock dividends (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Per Share (in dollars per share)	$ 1.975	$ 2	$ 1.9
Series A Preferred Stock One [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Percentage	8.25%	8.25%	8.25%
Dividend Per Share (in dollars per share)	$ 0.51563	$ 0.51563	$ 0.51563
Series A Preferred Stock Two [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Percentage	8.25%	8.25%	8.25%
Dividend Per Share (in dollars per share)	$ 0.51563	$ 0.51563	$ 0.51563
Series A Preferred Stock Three [Member] [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Percentage	8.25%	8.25%	8.25%
Dividend Per Share (in dollars per share)	$ 0.51563	$ 0.51563	$ 0.51563
Series A Preferred Stock Four [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Percentage	8.25%	8.25%	8.25%
Dividend Per Share (in dollars per share)	$ 0.51563	$ 0.51563	$ 0.51563
Series A Preferred Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Percentage	8.25%
Dividend Per Share (in dollars per share)	$ 2.06252	$ 2.06252	$ 2.06252
Series B Preferred Stock one [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Percentage	8.00%	8.00%	8.00%
Dividend Per Share (in dollars per share)	$ 0.5	$ 0.5	$ 0.5
Series B Preferred Stock Two [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Percentage	8.00%	8.00%	8.00%
Dividend Per Share (in dollars per share)	$ 0.5	$ 0.5	$ 0.5
Series B Preferred Stock Three [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Percentage	8.00%	8.00%	8.00%
Dividend Per Share (in dollars per share)	$ 0.5	$ 0.5	$ 0.5
Series B Preferred Stock Four [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Percentage	8.00%	8.00%	8.00%
Dividend Per Share (in dollars per share)	$ 0.5	$ 0.5	$ 0.5
Series B Preferred Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dividend Percentage	8.00%
Dividend Per Share (in dollars per share)	$ 2	$ 2	$ 2