Revenue from Contracts with Customers (Income Statement) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ 861,436	$ 730,413	$ 1,742,384	$ 1,473,593
Costs and expenses:
Total costs and expenses	807,972	688,162	1,627,631	1,346,836
Selling, general and administrative	282,456	283,073	577,894	573,645
Income before income taxes	53,464	42,251	114,753	126,757
Provision for income taxes	6,458	790	22,721	27,872
Net income from continuing operations	$ 47,006	$ 41,461	$ 92,032	$ 98,885
Basic earnings per share attributable to common stockholders:
Basic earnings per share attributable to common stockholders (in dollars per share)	$ 0.25	$ 0.22	$ 0.49	$ 0.53
Diluted earnings per share attributable to common stockholders (in dollars per share)	$ 0.25	$ 0.22	$ 0.49	$ 0.53
ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ 640,003		$ 1,293,790
Prior guidance | ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	852,781		1,724,060
Costs and expenses:
Total costs and expenses	810,342		1,629,841
Selling, general and administrative	285,587		582,017
Income before income taxes	42,439		94,221
Provision for income taxes	3,640		17,434
Net income from continuing operations	$ 38,799		$ 76,787
Basic earnings per share attributable to common stockholders:
Basic earnings per share attributable to common stockholders (in dollars per share)	$ 0.21		$ 0.41
Diluted earnings per share attributable to common stockholders (in dollars per share)	$ 0.22		$ 0.41
Total increase (decrease) | ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ 8,655		$ 18,324
Costs and expenses:
Total costs and expenses	(2,370)		(2,210)
Selling, general and administrative	(3,131)		(4,123)
Income before income taxes	11,025		20,532
Provision for income taxes	2,818		5,287
Net income from continuing operations	$ 8,207		$ 15,245
Basic earnings per share attributable to common stockholders:
Basic earnings per share attributable to common stockholders (in dollars per share)	$ 0.04		$ 0.08
Diluted earnings per share attributable to common stockholders (in dollars per share)	$ 0.03		$ 0.08
Equipment sales
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ 105,750	$ 121,384	$ 216,121	$ 245,887
Costs and expenses:
Cost of products and sales	47,106	51,506	93,160	96,122
Equipment sales | Prior guidance | ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	106,301		217,533
Costs and expenses:
Cost of products and sales	47,035		93,219
Equipment sales | Total increase (decrease) | ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	(551)		(1,412)
Costs and expenses:
Cost of products and sales	71		(59)
Software
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	91,702	81,319	167,996	154,165
Costs and expenses:
Cost of products and sales	26,459	23,361	50,514	46,515
Software | ASC 606
Costs and expenses:
Cost of products and sales	10,000		21,000
Software | Prior guidance | ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	82,054		147,329
Costs and expenses:
Cost of products and sales	25,769		48,542
Software | Total increase (decrease) | ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	9,648		20,667
Costs and expenses:
Cost of products and sales	690		1,972
Income before income taxes	9,000		19,000
Business services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	367,876	217,903	754,414	442,422
Costs and expenses:
Cost of products and sales	293,480	$ 153,063	590,879	$ 303,906
Business services | Prior guidance | ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	368,318		755,345
Business services | Total increase (decrease) | ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ (442)		$ (931)