Accounting Standards and Significant Accounting Policies - Adoption of New Accounting Standard (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
Statement of Income
Total revenues	$ 236,067	$ 214,706	$ 693,301	$ 623,198
Selling, general and administrative expenses	52,605	44,513	152,471	130,293
Amortization of customer and trade name intangibles	4,386	3,360	11,742	10,016
Operating income	37,585	44,282	113,565	118,411
Income tax (benefit) provision	(298)	5,521	(147)	14,820
Net income	$ 38,924	$ 38,836	$ 115,910	$ 103,375
Earnings per common share:
Basic (usd per share)	$ 1.00	$ 1.04	$ 3.01	$ 2.78
Diluted (usd per share)	$ 0.96	$ 0.99	$ 2.87	$ 2.63
Balance Sheet
Accounts receivable	$ 281,523		$ 281,523		$ 246,188
Prepaid expenses	32,790		32,790		32,206
Accounts receivable, long-term	12,966		12,966		12,107
Other intangibles, net	282,806		282,806		229,617
Total assets	1,864,622		1,864,622		1,611,351
Deferred revenue	326,421		326,421		298,613
Deferred income taxes	44,965		44,965		46,879
Retained earnings	740,373		740,373		624,463
Total liabilities and shareholders' equity	1,864,622		1,864,622		1,611,351
As Reported
Statement of Income
Total revenues		$ 214,146		$ 622,811
Selling, general and administrative expenses		44,656		131,249
Amortization of customer and trade name intangibles		3,492		10,413
Operating income		43,447		116,671
Income tax (benefit) provision		5,259		14,308
Net income		$ 38,263		$ 102,147
Earnings per common share:
Basic (usd per share)		$ 1.02		$ 2.74
Diluted (usd per share)		$ 0.97		$ 2.60
Balance Sheet
Accounts receivable					227,127
Prepaid expenses					27,252
Accounts receivable, long-term					7,536
Other intangibles, net					236,444
Total assets					1,589,592
Deferred revenue					309,461
Deferred income taxes					38,914
Retained earnings					599,821
Total liabilities and shareholders' equity					1,589,592
Accounting Standards Update 2014-09 | Adjustments
Statement of Income
Total revenues		$ 560		$ 387
Selling, general and administrative expenses		(143)		(956)
Amortization of customer and trade name intangibles		(132)		(397)
Operating income		835		1,740
Income tax (benefit) provision		262		512
Net income		573		1,228
Balance Sheet
Accounts receivable					19,061
Prepaid expenses					4,954
Accounts receivable, long-term					4,571
Other intangibles, net					(6,827)
Total assets					21,759
Deferred revenue					(10,848)
Deferred income taxes					7,965
Retained earnings					24,642
Total liabilities and shareholders' equity					$ 21,759
Software licenses and royalties
Statement of Income
Total revenues	22,444	22,762	67,620	63,826
Software licenses and royalties | As Reported
Statement of Income
Total revenues		19,842		55,172
Software licenses and royalties | Accounting Standards Update 2014-09 | Adjustments
Statement of Income
Total revenues		2,920		8,654
Subscriptions
Statement of Income
Total revenues	58,699	44,352	160,736	124,731
Subscriptions | As Reported
Statement of Income
Total revenues		44,840		125,889
Subscriptions | Accounting Standards Update 2014-09 | Adjustments
Statement of Income
Total revenues		(488)		(1,158)
Software services
Statement of Income
Total revenues	48,199	46,045	144,812	134,401
Software services | As Reported
Statement of Income
Total revenues		47,479		139,869
Software services | Accounting Standards Update 2014-09 | Adjustments
Statement of Income
Total revenues		(1,434)		(5,468)
Maintenance
Statement of Income
Total revenues	96,215	91,847	286,188	266,965
Maintenance | As Reported
Statement of Income
Total revenues		92,285		268,556
Maintenance | Accounting Standards Update 2014-09 | Adjustments
Statement of Income
Total revenues		(438)		(1,591)
Appraisal services
Statement of Income
Total revenues	5,544	6,290	16,470	19,268
Appraisal services | As Reported
Statement of Income
Total revenues		6,290		19,268
Appraisal services | Accounting Standards Update 2014-09 | Adjustments
Statement of Income
Total revenues		0		0
Hardware and other
Statement of Income
Total revenues	$ 4,966	3,410	$ 17,475	14,007
Hardware and other | As Reported
Statement of Income
Total revenues		3,410		14,057
Hardware and other | Accounting Standards Update 2014-09 | Adjustments
Statement of Income
Total revenues		$ 0		$ (50)