Segment Information - Segment (Details) ¥ in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($) segment	Dec. 31, 2019 CNY (¥) segment	Dec. 31, 2018 CNY (¥)	Dec. 31, 2017 CNY (¥)
Number of operating segments	2	2
Net revenues	$ 212,372	¥ 1,478,493	¥ 1,145,517	¥ 847,993
Cost of revenues	(63,191)	(439,923)	(410,908)	(314,121)
Gross profit	$ 149,181	¥ 1,038,570	¥ 734,609	¥ 533,872
Gross margin (as a percent)	70.20%	70.20%	64.10%	63.00%
Sales and marketing expenses	$ (113,849)	¥ (792,591)	¥ (731,233)	¥ (657,065)
Product development expenses	(22,624)	(157,505)	(185,000)	(223,202)
General and administrative expenses	(28,158)	(196,029)	(223,057)	(224,395)
Operating expenses	(164,631)	(1,146,125)	(1,139,290)	(1,104,662)
Loss from operations	(15,450)	(107,555)	(404,681)	(570,790)
One-on-one offerings
Net revenues	196,171	1,365,706	1,044,769	835,649
Cost of revenues	(55,458)	(386,085)	(342,927)	(293,763)
Gross profit	$ 140,713	¥ 979,621	¥ 701,842	¥ 541,886
Gross margin (as a percent)	71.70%	71.70%	67.20%	64.80%
Sales and marketing expenses	$ (106,009)	¥ (738,010)	¥ (647,314)	¥ (619,966)
Product development expenses	(19,864)	(138,291)	(139,240)	(202,129)
General and administrative expenses	(25,655)	(178,606)	(186,983)	(213,880)
Operating expenses	(151,528)	(1,054,907)	(973,537)	(1,035,975)
Loss from operations	(10,815)	(75,286)	(271,695)	(494,089)
Small class offerings
Net revenues	16,201	112,787	100,748	12,344
Cost of revenues	(7,733)	(53,838)	(67,981)	(20,358)
Gross profit	$ 8,468	¥ 58,949	¥ 32,767	¥ (8,014)
Gross margin (as a percent)	52.30%	52.30%	32.50%	(64.90%)
Sales and marketing expenses	$ (7,840)	¥ (54,581)	¥ (83,919)	¥ (37,099)
Product development expenses	(2,760)	(19,214)	(45,760)	(21,073)
General and administrative expenses	(2,503)	(17,423)	(36,074)	(10,515)
Operating expenses	(13,103)	(91,218)	(165,753)	(68,687)
Loss from operations	$ (4,635)	¥ (32,269)	¥ (132,986)	¥ (76,701)

Represents the gross margin percentage.

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

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