Deferred compensation (2017 Management Incentive Plan) (Details)			1 Months Ended	6 Months Ended	9 Months Ended
		Mar. 21, 2017 $ / shares shares	Aug. 31, 2017 USD ($) Tranche shares	Sep. 30, 2017 $ / shares shares	Sep. 30, 2017 $ / shares shares	Aug. 09, 2017 $ / shares shares
Common Class A
Stock-Based Compensation [Line Items]
Stock par value | $ / shares				$ 0.01	$ 0.01
2017 Management Incentive Plan
Shares
Awards vested					0
Repurchases of vested shares					0
2017 Management Incentive Plan | Time Shares
Stock-Based Compensation [Line Items]
Award vesting period (in years)			3 years
Award vesting period beginning date			Apr. 01, 2018
2017 Management Incentive Plan | Performance Shares
Stock-Based Compensation [Line Items]
Award vesting period (in years)			3 years
Award vesting period beginning date			Dec. 31, 2017
Number of tranches | Tranche			3
Shares based awards expense recognized | $			$ 0
2017 Management Incentive Plan | Employees and Board of Directors
Stock-Based Compensation [Line Items]
Granted (in shares)			1,796,943
2017 Management Incentive Plan | Employees | Time Shares
Stock-Based Compensation [Line Items]
Percentage of comprised shares for granted award			75.00%
2017 Management Incentive Plan | Employees | Performance Shares
Stock-Based Compensation [Line Items]
Percentage of comprised shares for granted award			25.00%
2017 Management Incentive Plan | Common Class A
Stock-Based Compensation [Line Items]
Stock par value | $ / shares						$ 0.01
Shares issued or reserved for issuance						3,388,832
2017 Management Incentive Plan | Common Class A | Time Shares
Stock-Based Compensation [Line Items]
Granted (in shares)	[1]			1,376,481
Weighted average grant date fair value
Unvested and outstanding at beginning of period ($ per share) | $ / shares				$ 0
Granted ($ per share) | $ / shares	[1]			20.05
Unvested and outstanding at end of period ($ per share) | $ / shares		$ 0		$ 20.05	$ 20.05
Shares
Unvested and outstanding at beginning of period (in shares)				0
Unvested and outstanding at end of period (in shares)		0		1,376,481	1,376,481
2017 Management Incentive Plan | Common Class A | Performance Shares
Stock-Based Compensation [Line Items]
Granted (in shares)	[1]			420,462
Weighted average grant date fair value
Unvested and outstanding at beginning of period ($ per share) | $ / shares				$ 0
Granted ($ per share) | $ / shares	[1]			20.05
Unvested and outstanding at end of period ($ per share) | $ / shares		$ 0		$ 20.05	$ 20.05
Shares
Unvested and outstanding at beginning of period (in shares)				0
Unvested and outstanding at end of period (in shares)		0		420,462	420,462
Common Stock | 2017 Management Incentive Plan
Stock-Based Compensation [Line Items]
Percentage of shares authorized for issuance		7.00%

[1] Includes 280,308 Performance Shares attributable to 2018 and 2019 performance conditions and 70,077 Performance Shares attributable to 2017 conditions where determination of accomplishment is discretionary. Under accounting guidance, a grant date has not been established for all these awards.