Stockholders' equity (Summary of Changes in Common Shares Outstanding) (Details) - shares		3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 21, 2017	Mar. 21, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		1,392,706	44,982,142	46,390,513	46,827,762
Issuance of restricted stock (in shares)					55,600
Conversion of Class B shares (in shares)					0
Repurchase of common stock (in shares)				(209,852)	(261,103)
Stock-based compensation (in shares)			1,853,236	2,002,173
Restricted stock forfeited (in shares)		(1,454)		(316,821)	(231,746)
Restricted stock canceled (in shares)		(8,964)	(7,616)
Cancellation of Predecessor equity (in shares)	(1,382,288)
Issuance of Successor common stock - rights offering (in shares)	4,197,210
Issuance of Successor common stock - backstop premium (in shares)	367,030
Issuance of Successor common stock - settlement of claims (in shares)	40,417,902			76,217
Balance at end of period (in shares)	44,982,142	44,982,142	46,827,762	47,942,230	46,390,513
Predecessor
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)			1,382,288
Balance at end of period (in shares)	1,382,288	1,382,288
Class A
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		333,686	37,110,630	46,390,513	38,956,250
Issuance of restricted stock (in shares)					55,600
Conversion of Class B shares (in shares)					7,871,512
Repurchase of common stock (in shares)				(209,852)	(261,103)
Stock-based compensation (in shares)			1,853,236	2,002,173
Restricted stock forfeited (in shares)		(1,454)		(316,821)	(231,746)
Restricted stock canceled (in shares)		(8,964)	(7,616)
Cancellation of Predecessor equity (in shares)	(323,268)
Issuance of Successor common stock - rights offering (in shares)	4,197,210
Issuance of Successor common stock - backstop premium (in shares)	367,030
Issuance of Successor common stock - settlement of claims (in shares)	32,546,390			76,217
Balance at end of period (in shares)	37,110,630	37,110,630	38,956,250	47,942,230	46,390,513
Class A | Predecessor
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)			323,268
Balance at end of period (in shares)	323,268	323,268
Class B
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		344,859	7,871,512	0	7,871,512
Conversion of Class B shares (in shares)					(7,871,512)
Cancellation of Predecessor equity (in shares)	(344,859)
Issuance of Successor common stock - settlement of claims (in shares)	7,871,512
Balance at end of period (in shares)	7,871,512	7,871,512	7,871,512	0	0
Class B | Predecessor
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)			344,859
Balance at end of period (in shares)	344,859	344,859
Class C
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		209,882	0	0
Cancellation of Predecessor equity (in shares)	(209,882)
Balance at end of period (in shares)	0	0		0	0
Class C | Predecessor
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)			209,882
Balance at end of period (in shares)	209,882	209,882
Class E
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		504,276	0	0
Cancellation of Predecessor equity (in shares)	(504,276)
Balance at end of period (in shares)	0	0		0	0
Class E | Predecessor
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)			504,276
Balance at end of period (in shares)	504,276	504,276
Class F
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		1	0	0
Cancellation of Predecessor equity (in shares)	(1)
Balance at end of period (in shares)	0	0		0	0
Class F | Predecessor
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)			1
Balance at end of period (in shares)	1	1
Class G
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		2	0	0
Cancellation of Predecessor equity (in shares)	(2)
Balance at end of period (in shares)	0	0		0	0
Class G | Predecessor
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)			2
Balance at end of period (in shares)	2	2