Note 3 - Adoption of New Revenue Recognition Standard - Impact of ASC 606 Adoption (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounts receivable, net		$ 28,168		$ 28,168		$ 29,930
Prepaid and other current assets		16,694		16,694		10,898
Total Current Assets		290,329		290,329		223,911
Other noncurrent assets		11,452		11,452		10,955
Total Assets		2,267,948		2,267,948		2,204,632
Accounts payable and accrued liabilities		101,565		101,565		117,855
Deferred revenue		18,681		18,681		15,008
Total Current Liabilities		139,309		139,309		147,238
Deferred income taxes		225,780		225,780		207,154
Other noncurrent liabilities		10,476		10,476		13,111
Total Liabilities		1,522,613		1,522,613		1,528,185
Retained earnings		819,687		819,687		728,386
Total Stockholders' Equity		745,335		745,335		676,447
Total Liabilities and Stockholders' Equity		2,267,948		2,267,948		2,204,632
Revenues		268,268	$ 253,833	802,443	$ 702,258
Selling, general and administrative		59,439	51,806	164,584	149,156
Total costs and expenses		204,949	192,918	603,795	525,272
Income from operations		63,319	60,915	198,648	176,986
Income before income taxes		49,362	47,174	156,514	143,821
Income tax provision		11,048	16,269	33,762	52,943
Net income		$ 38,314	$ 30,905	$ 122,752	$ 90,878
Basic (in dollars per share)		$ 6.75	$ 5.44	$ 21.58	$ 16
Diluted (in dollars per share)		$ 6.70	$ 5.37	$ 21.44	$ 15.82
Comprehensive income		$ 38,315	$ 30,906	$ 122,754	$ 90,883
Increase in accounts receivable, net				18,626	9,820
Increase in deferred revenue				3,673	119
Change in other noncurrent assets and liabilities, net				(2,921)	2,487
Net cash provided by operating activities				$ 307,617	219,788
Previously Reported [Member]
Accounts receivable, net						51,141
Prepaid and other current assets						8,160
Total Current Assets						242,384
Other noncurrent assets						6,179
Total Assets						2,218,329
Accounts payable and accrued liabilities						117,963
Deferred revenue						38,266
Total Current Liabilities						170,604
Deferred income taxes						205,636
Other noncurrent liabilities						9,991
Total Liabilities						1,546,913
Retained earnings						723,354
Total Stockholders' Equity						671,416
Total Liabilities and Stockholders' Equity						2,218,329
Revenues	[1]		253,846		702,315
Selling, general and administrative	[1]		51,968		149,218
Total costs and expenses	[1]		193,080		525,334
Income from operations	[1]		60,766		176,981
Income before income taxes	[1]		47,025		143,816
Income tax provision	[1]		16,213		52,941
Net income	[1]		$ 30,812		$ 90,875
Basic (in dollars per share)	[1]		$ 5.42		$ 16
Diluted (in dollars per share)	[1]		$ 5.35		$ 15.82
Comprehensive income	[1]		$ 30,813		$ 90,880
Increase in accounts receivable, net					9,818
Increase in deferred revenue					452
Change in other noncurrent assets and liabilities, net					2,159
Net cash provided by operating activities					219,788
Restatement Adjustment [Member]
Accounts receivable, net						(21,211)
Prepaid and other current assets						2,738
Total Current Assets						(18,473)
Other noncurrent assets						4,776
Total Assets						(13,697)
Accounts payable and accrued liabilities						(108)
Deferred revenue						(23,258)
Total Current Liabilities						(23,366)
Deferred income taxes						1,518
Other noncurrent liabilities						3,120
Total Liabilities						(18,728)
Retained earnings						5,032
Total Stockholders' Equity						5,031
Total Liabilities and Stockholders' Equity						$ (13,697)
Revenues			(13)		(57)
Selling, general and administrative			(162)		(62)
Total costs and expenses			(162)		(62)
Income from operations			149		5
Income before income taxes			149		5
Income tax provision			56		2
Net income			$ 93		$ 3
Basic (in dollars per share)			$ 0.02
Diluted (in dollars per share)			$ 0.02
Comprehensive income			$ 93		$ 3
Increase in accounts receivable, net					2
Increase in deferred revenue					(333)
Change in other noncurrent assets and liabilities, net					328
Net cash provided by operating activities

[1] Refer to note 2 for details regarding the revision.