Deferred compensation (Equity Awards) (Details) - $ / shares	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Common Class A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, par value (in dollars per share)		$ 0.01
Management Incentive Plan | Common Class A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued or reserved for issuance (in shares)		3,500,000