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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Name:	us-gaap_ConversionOfStockLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Earnings per share - Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jan. 26, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Conversion of Stock [Line Items]
Conversion of stock, additional cash issued to Class B common stock holders		$ 0	$ 0	$ 12,000	$ 0
Conversion, Class B Common Stock to Common Stock
Conversion of Stock [Line Items]
Conversion of stock, additional cash issued to Class B common stock holders	$ 12,000			$ 12,000