Income taxes - Schedule of Reconciliation of U.S. Statutory Income Tax Rate to Company's Effective Tax Rate (Detail)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S. statutory tax rate (in percentage)	21.00%	35.00%	35.00%
State and local taxes, net of federal benefit (in percentage)	5.90%	1.00%	4.90%
State rate change impact on deferred taxes (in percentage)	(3.40%)	0.80%	(1.40%)
Federal rate change impact on deferred taxes (in percentage)	0.00%	77.80%	0.00%
Tax benefit arrangement liability adjustment (in percentage)	0.80%	(25.80%)	0.00%
Foreign tax rate differential (in percentage)	0.20%	0.00%	(0.30%)
Withholding taxes and other (in percentage)	(0.30%)	0.10%	0.00%
Reserve for uncertain tax position (in percentage)	(0.20%)	0.10%	3.10%
Income attributable to non-controlling interests (in percentage)	(2.30%)	(1.90%)	(20.50%)
Effective tax rate (in percentage)	21.70%	87.10%	20.80%

Effective income tax rate reconciliation impact of federal tax rate change on deferred taxes.

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- Definition

Effective income tax rate reconciliation impact of state tax rate change on deferred taxes.

+ References

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- Definition

Effective income tax rate reconciliation reserve for uncertain tax positions.

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Effective income tax rate reconciliation tax benefit arrangement liability adjustment.

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- Definition

Effective Income Tax Rate Reconciliation, Withholding Taxes And Other, Percent

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(2))
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-Topic 740
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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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-Publisher FASB
-Name Accounting Standards Codification
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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to statutory income tax expense (benefit) outside of the country of domicile.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to noncontrolling interest income (loss) exempt from income taxes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
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-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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