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Purchase of common shares.

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Share-based compensation arrangement by share-based payment award vesting start date.

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Termination fee settled in shares.

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
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-Subparagraph (SAB Topic 14.F)
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Represents the compensation cost capitalized during the period arising from equity-based compensation arrangements (for example, shares of stock, units, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Description of award terms as to how many shares or portion of an award are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, to sell the shares, and be entitled to the cash proceeds of such sale. For example, vesting may be expressed as being 25 percent of the shares under option on each anniversary of the grant date.

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General descriptive information regarding an arrangement under which employees and directors receive: (a) awards of equity shares, equity share options, or other equity instruments; or (b) the entity incurs liabilities to them: (1) in amounts based, at least in part, on the price of the entity's shares or other equity instruments; or (2) that require or may require settlement by issuance of the entity's shares. Such an arrangement is usually provided to employees and directors to compensate them, provide performance incentives to them, and to attract or retain their services. May also include pertinent information particular to a plan that is not elsewhere specified in the taxonomy.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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Date the equity-based award expires, in CCYY-MM-DD format.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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- Definition

Gross number of share options (or share units) granted during the period.

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No definition available.

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Number of share instruments newly issued under a share-based compensation plan.

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Per share or per unit amount of equity securities issued.

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Value, after forfeitures, of stock or other type of equity granted of any equity-based compensation plan other than an employee stock ownership plan (ESOP).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Share-based compensation - Additional Information (Detail) - USD ($)	1 Months Ended					12 Months Ended
	Dec. 31, 2018	Jan. 31, 2018	Aug. 31, 2017	Jul. 31, 2017	May 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total shares of common stock reserved for issuance under the plan							5,000,000		3,000,000
Remaining shares left for issuance under this plan	2,187,420					2,187,420	2,952,896
Share-based compensation expenses						$ 2,989,000	$ 10,400,000	$ 6,228,000
Total unrecognized compensation costs relating to unvested share-based compensation awards	$ 1,474,000					$ 1,474,000	$ 4,178,000
Expected to be recognized over a weighted average period						24 months	24 months
Transaction Fees [Member] \| Vessels [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Transaction fees capitalized to vessels						$ 2,326,000	$ 2,262,000	6,317,000
Arrangement Fees [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Arrangement fees capitalized to deferred financing fees							1,872,000	$ 7,598,000
Restricted shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period						1 year
Total fair value of shares vested						$ 1,018,000	$ 880,000
Granted, stock units						664,326	107,270	56,861
Weighted average grant date fair value						$ 7.68	$ 8.97	$ 15.48
Phantom share units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period						3 years
Outstanding phantom share units vested and available for exchange	537,002					537,002	587,001
Granted, stock units						30,000	90,000	60,000
Weighted average grant date fair value						$ 6.86	$ 6.85	$ 18.84
Restricted stock units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period						3 years
Granted, stock units						109,248	88,293	528,232
Retention requirement description						The restricted stock units generally vest over three years, in equal one-third amounts on each anniversary date of the date of the grant.
Expiration date						Jun. 30, 2018
Weighted average grant date fair value						$ 9.73	$ 5.93	$ 16.57
Stock options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, stock units						500,000	0	0
Weighted average grant date fair value						$ 7.20	$ 0	$ 0
Chief Executive Officer [Member] \| Restricted shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	5 years
Granted, stock units	500,000
Chief Executive Officer [Member] \| Restricted stock units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, stock units					479,714
Chief Executive Officer [Member] \| Stock options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period		5 years
Stock options, vesting description		The stock options vest equally on each of the first five anniversaries of the CEO’s start date in January 2018 and expire on January 8, 2028. As at December 31, 2018, no stock options have been vested.
Stock options, vesting start date		Jan. 31, 2018
Expiration date		Jan. 08, 2028
Chief Executive Officer [Member] \| Performance Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, stock units					786,147
Weighted average grant date fair value					$ 10.23
Director And Officer \| Restricted stock units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, stock units						12,441
Weighted average grant date fair value						$ 7.28
Class A Common Shares [Member] \| Transaction Fees [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Transaction fees settled in Class A common shares							$ 2,231,000	$ 3,159,000
Class A Common Shares [Member] \| Arrangement Fees [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Transaction fees settled in Class A common shares							1,872,000	$ 3,799,000
Class A Common Shares [Member] \| Chairman [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expenses							6,920,000,000
Common shares granted				1,000,000
Purchase of common shares by chairman			1,000,000
Common shares purchased, price per share			$ 6.00
Class A Common Shares [Member] \| Chief Executive Officer [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, shares Issued to CEO					200,000
Class A Common Shares [Member] \| Chief Executive Officer [Member] \| Stock options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issuable upon exercise of options		500,000
Stock options, exercise price		$ 7.20
Class A Common Shares [Member] \| Director [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Termination fee paid							$ 6,250,000
Termination fee paid, shares							945,537