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Summary of Significant Accounting Policies - Summary of Reflection of New Adopted Standards in Selected Consolidated Statement of Operations Line Items (Detail) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Costs and expenses: |  |  |  |  |  |  |  |  |  |  |
Sales and marketing |  |  |  |  |  |  |  |  | $ 150,512 | $ 119,258 | $ 92,554
General and administrative |  |  |  |  |  |  |  |  | 91,647 | 69,046 | 47,826
Operating income | $ 19,434 | $ 11,437 | $ 9,089 | $ 38,665 | $ 12,681 | $ 582 | $ 16,004 | $ 28,704 | 78,625 | 57,971 | 34,435
Net income |  |  |  |  |  |  |  |  | $ 66,807 | $ 43,840 | $ 20,945
Earnings per share, basic | $ 0.22 | $ 0.24 | $ 0.24 | $ 0.44 | $ 0.15 | $ 0.11 | $ 0.18 | $ 0.32 | $ 1.15 | $ 0.76 | $ 0.37
Earnings per share, diluted | $ 0.22 | $ 0.24 | $ 0.24 | $ 0.43 | $ 0.15 | $ 0.10 | $ 0.18 | $ 0.31 | $ 1.13 | $ 0.74 | $ 0.36
Accounting Standards Update 2014-09 [Member] |  |  |  |  |  |  |  |  |  |  |
Costs and expenses: |  |  |  |  |  |  |  |  |  |  |
Sales and marketing |  |  |  |  |  |  |  |  | $ 110,846 | $ 85,361 |
General and administrative |  |  |  |  |  |  |  |  | 80,228 | 59,174 |
Operating income |  |  |  |  |  |  |  |  | 129,710 | 101,740 |
Net income |  |  |  |  |  |  |  |  | $ 123,486 | $ 70,421 |
Earnings per share, basic |  |  |  |  |  |  |  |  | $ 2.13 | $ 1.21 |
Earnings per share, diluted |  |  |  |  |  |  |  |  | $ 2.10 | $ 1.19 |

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- Definition The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 55 -Paragraph 52 -URI http://asc.fasb.org/extlink&oid=96948231&loc=d3e4984-109258 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(19)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96947427&loc=d3e1252-109256 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(21)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(23)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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