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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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value

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Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Measurement Period For Expected Volatility Rate

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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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- Definition

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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- Definition

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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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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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 grantees can acquire the shares reserved for issuance under the stock option plan.

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- Definition

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 (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
	Mar. 01, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Share-based compensation
Capitalized share-based compensation expense		$ 50	$ 30	$ 90	$ 70
Income tax benefit from compensation expense		$ 700	$ 1,100	$ 1,400	1,900
Equity and Incentive Plan
Share-based compensation
Number of additional shares authorized	1,500,000
Shares available for future issuance		3,500,000		3,500,000
Number of share issuable upon vesting and achievement of performance goals		800,000		800,000
Nonemployee Director Stock Plan
Share-based compensation
Shares available for future grant		141,044		141,044
Income tax benefit from compensation expense				$ 300	$ 300
Nonemployee director, shares granted				28,246	33,170
Nonemployee director, fair value of grants				$ 800	$ 800
Long-term Incentive 2012 to 2014 Plan Linked to Total Return to Shareholders
Share-based compensation
Award requisite service period				3 years
Stock appreciation rights (SARs)
Share-based compensation
Options outstanding		0		0			80,000
Options, weighted average exercise price (in dollars per share)							$ 26.18
Shares underlying SARS exercised		0	0	80,000	0
Weighted-average price of shares exercised		$ 0.00	$ 0.00	$ 26.18	$ 0.00
Aggregate intrinsic value of options outstanding		$ 0	$ 0	$ 502	$ 0
Tax benefit realized for the deduction of exercises		0	$ 0	82	0
Restricted stock units
Share-based compensation
Unrecognized share based compensation		$ 5,600		$ 5,600
Weighted average period for recognition of unrecognized compensation cost				2 years 9 months 18 days
Fair value measurement of vested units and related dividends				$ 3,000	4,000
Income tax benefit from compensation expense				$ 800	$ 1,300
Nonvested equity instruments other than options outstanding		252,302	264,326	252,302	264,326	261,691	261,235	332,158	288,151
Nonvested equity instruments other than options, weighted average grant date fair value		$ 28.35	$ 25.74	$ 28.35	$ 25.74	$ 28.33	$ 25.77	$ 25.04	$ 25.17
Nonemployee director, shares granted		788	0	85,082	115,036
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		$ 1,200		$ 1,200
Weighted average period for recognition of unrecognized compensation cost				1 year
Measurement period for total return to shareholders				3 years
Stock volatility measurement period				3 years
Nonvested equity instruments other than options outstanding		163,423	257,956	163,423	257,956	168,777	257,956	258,243	232,127
Nonvested equity instruments other than options, weighted average grant date fair value		$ 27.63	$ 28.45	$ 27.63	$ 28.45	$ 27.63	$ 28.45	$ 28.46	$ 32.88
Nonemployee director, shares granted		0	731	0	97,524
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		$ 1,800		$ 1,800
Weighted average period for recognition of unrecognized compensation cost				1 year
Fair value measurement of vested units and related dividends				$ 4,700	$ 1,900
Income tax benefit from compensation expense				$ 1,800	$ 800
Nonvested equity instruments other than options outstanding		220,158	359,782	220,158	359,782	230,219	364,731	360,070	296,843
Nonvested equity instruments other than options, weighted average grant date fair value		$ 26.00	$ 26.01	$ 26.00	$ 26.01	$ 26.00	$ 26.01	$ 26.01	$ 26.14
Nonemployee director, shares granted		0	730	0	129,603
Long-term Incentive 2013 to 2015 Plan Linked to Total Return to Shareholders
Share-based compensation
Award requisite service period				3 years