Represents the number of years used in the calculation of total return to shareholders.

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Represents the high end of range of payout depending on achievement of the goals.

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Represents the low end of range of payout depending on achievement of the goals.

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Represents the number of shares that could be issued upon vesting of share-based payment awards and achievement of performance goals.

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Represents the number of authorized but unissued shares that could be issued under the equity-based compensation plan.

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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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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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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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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 additional shares authorized for issuance under an established share-based compensation plan.

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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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No definition available.

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Number of share options (or share units) exercised during the current period.

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Share-based compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 01, 2014	Jul. 31, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Share-based compensation
Share-based compensation expense			$ 1,000	$ 2,000	$ 2,000	$ 3,800
Capitalized share-based compensation expense				50		90
Income tax benefit from compensation expense			$ 400	$ 700	$ 700	$ 1,400
Stock appreciation rights (SARs)
Share-based compensation
Options outstanding			0		0		0
Shares underlying SARs exercised			0			80,000
Weighted-average price of shares exercised (in dollars per share)			$ 0.00			$ 26.18
Aggregate intrinsic value of options outstanding			$ 0			$ 502
Tax benefit realized for the deduction of exercises			0			82
Restricted stock units
Share-based compensation
Unrecognized share based compensation			5,500		$ 5,500
Weighted average period for recognition of unrecognized compensation cost					2 years 9 months 18 days
Fair value measurement of vested units and related dividends					$ 2,600	3,000
Income tax benefit from compensation expense					$ 900	800
Long-term Incentive Plan
Share-based compensation
Payment award, low end of range					0.00%
Payment award, high end of range					200.00%
LTIP linked to TRS
Share-based compensation
Unrecognized share based compensation			300		$ 300
Weighted average period for recognition of unrecognized compensation cost					6 months
Measurement period for total return to shareholders					3 years
Long-term Incentive 2014 to 2016 Plan Linked to Total Return to Shareholders
Share-based compensation
Award requisite service period					3 years
LTIP awards linked to other performance conditions
Share-based compensation
Unrecognized share based compensation			$ 500		$ 500
Weighted average period for recognition of unrecognized compensation cost					6 months
Fair value measurement of vested units and related dividends					$ 3,600	4,700
Income tax benefit from compensation expense					$ 1,400	1,800
Equity and Incentive Plan
Share-based compensation
Number of additional shares authorized	1,500,000
Shares available for future issuance			3,400,000		3,400,000
Number of share issuable upon vesting and achievement of performance goals			400,000		400,000
Nonemployee Director Stock Plan
Share-based compensation
Shares available for future grant			141,044		141,044
Nonemployee Director Stock Plan \| Stock Compensation Plan
Share-based compensation
Fair value measurement of vested units and related dividends					$ 0	800
Income tax benefit from compensation expense					$ 0	$ 300
Nonemployee Director Stock Plan \| Stock Compensation Plan \| Subsequent Event
Share-based compensation
Share-based compensation expense		$ 200