Common stock (Details 1) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Treasury Stock Value outstanding	$ 82,008us-gaap_TreasuryStockValue
Repurchase of common stock	640	27
Restricted stock issued - directors	0	0	0
Treasury Stock Value outstanding	77,627us-gaap_TreasuryStockValue	82,008us-gaap_TreasuryStockValue
Common Stock [Member]
Class of Stock [Line Items]
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 Class A [Member]
Class of Stock [Line Items]
Treasury Stock Value outstanding	10,708	10,708	10,708
Treasury Stock, outstanding (in shares)	318,140	318,140	318,140
Repurchase of common stock	0	0
Repurchase of common stock (in shares)	0	0
Restricted stock issued - directors	0	0	0
Restricted stock issued - directors (in shares)	0	0	0
Exercised stock options - compensation plans	0	0	0
Exercised stock options - compensation plans (in shares)	0	0	0
Restricted stock units - vested	0	0
Restricted stock units - vested (in shares)	0	0	0
Treasury Stock Value outstanding	10,708	10,708	10,708
Treasury Stock, outstanding (in shares)	318,140	318,140	318,140
Common Class B [Member]
Class of Stock [Line Items]
Treasury Stock Value outstanding	15,655	15,655	15,655
Treasury Stock, outstanding (in shares)	568,010	568,010	568,010
Repurchase of common stock	587	0
Repurchase of common stock (in shares)	21,164	0
Restricted stock issued - directors	0	0	0
Restricted stock issued - directors (in shares)	0	0	0
Exercised stock options - compensation plans	0	0	0
Exercised stock options - compensation plans (in shares)	0	0	0
Restricted stock units - vested	0	0	0
Restricted stock units - vested (in shares)	0	0	0
Treasury Stock Value outstanding	16,242	15,655	15,655
Treasury Stock, outstanding (in shares)	589,174	568,010	568,010
Common Class E [Member]
Class of Stock [Line Items]
Treasury Stock Value outstanding	55,645	65,089	89,254
Treasury Stock, outstanding (in shares)	2,520,522	2,948,508	3,961,748
Repurchase of common stock	53	27
Repurchase of common stock (in shares)	2,110	1,083
Restricted stock issued - directors	(629)	(629)	(771)
Restricted stock issued - directors (in shares)	(28,500)	(28,500)	(32,317)
Exercised stock options - compensation plans	(2,460)	(6,094)	(21,361)
Exercised stock options - compensation plans (in shares)	(111,427)	(276,079)	(895,674)
Restricted stock units - vested	(1,932)	(2,748)	(2,033)
Restricted stock units - vested (in shares)	(87,519)	(124,490)	(85,249)
Treasury Stock Value outstanding	50,677	55,645	65,089
Treasury Stock, outstanding (in shares)	2,295,186	2,520,522	2,948,508
Treasury stock
Class of Stock [Line Items]
Treasury Stock Value outstanding	82,008	91,452	115,617
Treasury Stock, outstanding (in shares)	3,406,672	3,834,658	4,847,898
Repurchase of common stock	640	27
Repurchase of common stock (in shares)	23,274	1,083
Restricted stock issued - directors	(629)	(629)	(771)
Restricted stock issued - directors (in shares)	(28,500)	(28,500)	(32,317)
Exercised stock options - compensation plans	(2,460)	(6,094)	(21,361)
Exercised stock options - compensation plans (in shares)	(111,427)	(276,079)	(895,674)
Restricted stock units - vested	(1,932)	(2,748)	(2,033)
Restricted stock units - vested (in shares)	(87,519)	(124,490)	(85,249)
Treasury Stock Value outstanding	$ 77,627	$ 82,008	$ 91,452
Treasury Stock, outstanding (in shares)	3,202,500	3,406,672	3,834,658

Restricted stock units vested during the period.

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Restricted stock units value vested during 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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Number of share options (or share units) exercised during the current period.

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Value of stock related to Restricted Stock Awards issued during the period, net of the stock value of such awards forfeited.

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Value of stock issued as a result of the exercise of stock options.

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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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Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends.

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The amount allocated to treasury stock. Treasury stock is common and preferred shares of an entity that were issued, repurchased by the entity, and are held in its treasury.

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