New accounting pronouncements (other standards) (Details) - USD ($) $ in Thousands		3 Months Ended									12 Months Ended
		Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019	Jan. 01, 2018
Payments for acquisitions, net of cash acquired	[1]										$ (214,258)	$ (139,223)	$ (239,664)
Purchases of customer funds marketable securities											(7,807)	(7,737)	(7,869)
Proceeds from customer funds and corporate marketable securities											7,807	11,237	9,504
Net cash used by investing activities											(275,414)	(180,891)	(279,511)
Net change in customer funds obligations											20,279	(6,007)	1,723
Net cash (used) provided by financing activities,											(39,825)	(182,956)	5,998
Effect of exchange rate change on cash, cash equivalents, restricted cash and restricted cash equivalents											(7,636)	5,370	2,363
Net change in cash, cash equivalents, restricted cash and restricted cash equivalents											16,440	(20,046)	48,162
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of year		$ 145,259	$ 126,114	$ 145,303	$ 146,479	$ 128,819	$ 115,172	$ 105,479	$ 160,000	$ 148,865	128,819	148,865	100,703
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of year (Note 3)			145,259	126,114	145,303	146,479	128,819	115,172	105,479	160,000	145,259	128,819	148,865
Total cost of revenue											(791,748)	(742,707)	(667,813)
Gross profit			309,522	295,556	298,043	303,156	303,935	304,598	305,864	308,452	1,206,277	1,222,849	1,181,249
Selling, general and administrative expense											(854,000)	(830,231)	(807,238)
Operating income											231,221	329,176	366,887
Other income											8,522	5,010	3,659
Net income			57,170	(31,083)	60,207	63,336	84,709	28,801	59,579	57,066	149,630	230,155	229,382
Impact of Tax Cuts and Jobs Act			$ (582)	$ (1,249)	$ 441	(310)	(20,500)	$ 0	$ 0	0	(1,700)	(20,500)
Retained earnings [Member]
Net income											149,630	230,155	229,382
Product [Member]
Total cost of revenue											(547,640)	(529,638)	(534,906)
Service [Member]
Total cost of revenue											(244,108)	(213,069)	(132,907)
As previously reported [Member]
Payments for acquisitions, net of cash acquired													(270,939)
Purchases of customer funds marketable securities												0	0
Proceeds from customer funds and corporate marketable securities												3,500	1,635
Net cash used by investing activities												(180,891)	(310,786)
Net change in customer funds obligations												0	0
Net cash (used) provided by financing activities,												(176,949)	4,275
Effect of exchange rate change on cash, cash equivalents, restricted cash and restricted cash equivalents												2,075	1,346
Net change in cash, cash equivalents, restricted cash and restricted cash equivalents												(17,334)	14,147
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of year						59,240				76,574	59,240	76,574	62,427
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of year (Note 3)							59,240					59,240	76,574
Total cost of revenue												(742,090)	(667,241)
Gross profit												1,223,466	1,181,821
Selling, general and administrative expense												(828,832)	(805,970)
Operating income												331,192	368,727
Other income												2,994	1,819
Net income												230,155	229,382
As previously reported [Member] | Product [Member]
Total cost of revenue												(529,088)	(534,390)
As previously reported [Member] | Service [Member]
Total cost of revenue												(213,002)	(132,851)
Accounting Standards Update No. 2016-01 [Member]
Effect of adoption											0
Accounting Standards Update No. 2016-16 [Member] | Retained earnings [Member]
Cumulative effect of adoption															$ 0
Accounting Standards Update No. 2016-18 [Member] | Effect of adoption [Member]
Payments for acquisitions, net of cash acquired													31,275
Purchases of customer funds marketable securities												(7,737)	(7,869)
Proceeds from customer funds and corporate marketable securities												7,737	7,869
Net cash used by investing activities												0	31,275
Net change in customer funds obligations												(6,007)	1,723
Net cash (used) provided by financing activities,												(6,007)	1,723
Effect of exchange rate change on cash, cash equivalents, restricted cash and restricted cash equivalents												3,295	1,017
Net change in cash, cash equivalents, restricted cash and restricted cash equivalents												(2,712)	34,015
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of year						$ 69,579				$ 72,291	69,579	72,291	38,276
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of year (Note 3)							69,579					69,579	72,291
Accounting Standards Update No. 2017-07 [Member] | Effect of adoption [Member]
Total cost of revenue												(617)	(572)
Gross profit												(617)	(572)
Selling, general and administrative expense												(1,399)	(1,268)
Operating income												(2,016)	(1,840)
Other income												2,016	1,840
Net income												0	0
Accounting Standards Update No. 2017-07 [Member] | Effect of adoption [Member] | Product [Member]
Total cost of revenue												(550)	(516)
Accounting Standards Update No. 2017-07 [Member] | Effect of adoption [Member] | Service [Member]
Total cost of revenue												(67)	$ (56)
Accounting Standards Update No. 2018-02 [Member]
Cumulative effect of adoption							0					0
Accounting Standards Update No. 2018-02 [Member] | Accumulated other comprehensive loss [Member]
Cumulative effect of adoption							(6,867)					(6,867)			(6,867)
Accounting Standards Update No. 2018-02 [Member] | Retained earnings [Member]
Cumulative effect of adoption							$ 6,867					$ 6,867			$ 6,867
Accounting Standards Update No. 2018-05 [Member]
Impact of Tax Cuts and Jobs Act											$ (1,700)
Accounting Standards Update No. 2016-02 [Member] | Forecast [Member]
Lease, practical expedient, short-term leases		true
Accounting Standards Update No. 2016-02 [Member] | Forecast [Member] | Minimum [Member]
Cumulative effect of adoption														$ 45,000

[1] ) Cash and cash equivalents acquired were $1,645 during 2018, $27,299 during 2017 and $146 during 2016.