Common stock (Details)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common Stock [Member]
Class of Stock [Line Items]
Common stock, Authorized	280,000,000	280,000,000	280,000,000
Common stock, Outstanding (in shares)	38,573,167	38,145,182	37,131,942
Conversions (in shares)	0	(1)	0
Repurchase of common stock (in shares)	(23,274)	(1,083)
Restricted stock issued - directors (in shares)	28,500	28,500	32,317
Exercised stock options - compensation plans (in shares)	111,427	276,079	895,674
Restricted stock units - vested (in shares)	87,519	124,490	85,249
Common stock, Outstanding (in shares)	38,777,339	38,573,167	38,145,182
Common Class A [Member]
Class of Stock [Line Items]
Common stock, Authorized	40,000,000	40,000,000	40,000,000
Common stock, Outstanding (in shares)	6,342,189	6,342,189	6,342,189
Conversions (in shares)	0	0	0
Repurchase of common stock (in shares)	0	0
Restricted stock issued - directors (in shares)	0	0	0
Exercised stock options - compensation plans (in shares)	0	0	0
Restricted stock units - vested (in shares)	0	0	0
Common stock, Outstanding (in shares)	6,342,189	6,342,189	6,342,189
Common Class B [Member]
Class of Stock [Line Items]
Common stock, Authorized	40,000,000	40,000,000	40,000,000
Common stock, Outstanding (in shares)	2,520,422	2,531,926	2,531,926
Conversions (in shares)	(20,208)	(11,504)	0
Repurchase of common stock (in shares)	(21,164)	0
Restricted stock issued - directors (in shares)	0	0	0
Exercised stock options - compensation plans (in shares)	0	0	0
Restricted stock units - vested (in shares)	0	0	0
Common stock, Outstanding (in shares)	2,479,050	2,520,422	2,531,926
Common Class E [Member]
Class of Stock [Line Items]
Common stock, Authorized	100,000,000	100,000,000	100,000,000
Common stock, Outstanding (in shares)	29,710,556	29,271,067	28,257,827
Conversions (in shares)	20,208	11,503	0
Repurchase of common stock (in shares)	(2,110)	(1,083)
Restricted stock issued - directors (in shares)	28,500	28,500	32,317
Exercised stock options - compensation plans (in shares)	111,427	276,079	895,674
Restricted stock units - vested (in shares)	87,519	124,490	85,249
Common stock, Outstanding (in shares)	29,956,100	29,710,556	29,271,067
Common Class F [Member]
Class of Stock [Line Items]
Common stock, Authorized	100,000,000	100,000,000	100,000,000
Common stock, Outstanding (in shares)	0	0	0
Conversions (in shares)	0	0	0
Repurchase of common stock (in shares)	0	0
Restricted stock issued - directors (in shares)	0	0	0
Exercised stock options - compensation plans (in shares)	0	0	0
Restricted stock units - vested (in shares)	0	0	0
Common stock, Outstanding (in shares)	0	0	0

Restricted stock units vested during the period.

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- Definition

The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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- Definition

The number of shares converted in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

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Number of shares issued during the period related to Restricted Stock Awards, net of any shares forfeited.

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- Definition

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

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Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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