Revenue from Contracts with Customers (Income Statement) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue:
Total Consolidated Revenue	$ 983,182	$ 836,640
Equipment sales	155,808	162,974
Software	81,616	77,867
Business services	386,538	224,519
Costs and expenses:
Total costs and expenses	910,090	740,091
Cost of equipment sales	78,751	69,562
Cost of software	25,353	25,308
Selling, general and administrative	312,108	304,847
Income before income taxes	73,092	96,549
Provision for income taxes	19,579	31,416
Net income	$ 53,513	$ 65,133
Earnings per share attributable to common stockholders:
Basic earnings per share attributable to common stockholders (in dollars per share)	$ 0.29	$ 0.35
Diluted earnings per share attributable to common stockholders (in dollars per share)	$ 0.28	$ 0.35
ASC 606
Revenue:
Total Consolidated Revenue	$ 983,182
Equipment sales	155,808
Software	81,616
Business services	386,538
Costs and expenses:
Total costs and expenses	910,090
Cost of equipment sales	78,751
Cost of software	25,353
Selling, general and administrative	312,108
Income before income taxes	73,092
Provision for income taxes	19,579
Net income	$ 53,513
Earnings per share attributable to common stockholders:
Basic earnings per share attributable to common stockholders (in dollars per share)	$ 0.29
Diluted earnings per share attributable to common stockholders (in dollars per share)	$ 0.28
Pro forma | ASC 606
Revenue:
Total Consolidated Revenue	$ 970,451
Equipment sales	153,607
Software	70,597
Business services	387,027
Costs and expenses:
Total costs and expenses	907,747
Cost of equipment sales	76,738
Cost of software	24,071
Selling, general and administrative	313,060
Income before income taxes	62,704
Provision for income taxes	16,931
Net income	$ 45,773
Earnings per share attributable to common stockholders:
Basic earnings per share attributable to common stockholders (in dollars per share)	$ 0.25
Diluted earnings per share attributable to common stockholders (in dollars per share)	$ 0.24
Total increase (decrease) | ASC 606
Revenue:
Total Consolidated Revenue	$ 12,731
Equipment sales	2,201
Software	11,019
Business services	(489)
Costs and expenses:
Total costs and expenses	2,343
Cost of equipment sales	2,013
Cost of software	1,282
Selling, general and administrative	(952)
Income before income taxes	10,388
Provision for income taxes	2,648
Net income	$ 7,740
Earnings per share attributable to common stockholders:
Basic earnings per share attributable to common stockholders (in dollars per share)	$ 0.04
Diluted earnings per share attributable to common stockholders (in dollars per share)	$ 0.04