Goodwill and Other Assets (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill	$ 1,379.3	$ 1,407.8	$ 1,289.0
Goodwill, Impairment Loss	(90.0)
Accumulated impairment losses	(380.7)	(290.7)	(290.7)
Beginning balance	1,117.1	998.3
Goodwill acquired		5.6
Goodwill adjustment		(2.9)
Currency translation adjustment	(28.5)	116.1
Ending balance	998.6	1,117.1
Eurasia Banking
Goodwill [Line Items]
Goodwill	570.4	585.3	513.0
Goodwill, Impairment Loss	(53.5)
Accumulated impairment losses	(222.2)	(168.7)	(168.7)
Beginning balance	416.6	344.3
Goodwill acquired		1.6
Goodwill adjustment		(1.1)
Currency translation adjustment	(14.9)	71.8
Ending balance	348.2	416.6
Americas Banking
Goodwill [Line Items]
Goodwill	422.1	426.6	425.4
Goodwill, Impairment Loss	0.0
Accumulated impairment losses	(122.0)	(122.0)	(122.0)
Beginning balance	304.6	303.4
Goodwill acquired		0.0
Goodwill adjustment		(1.0)
Currency translation adjustment	(4.5)	2.2
Ending balance	300.1	304.6
Retail
Goodwill [Line Items]
Goodwill	386.8	395.9	350.6
Goodwill, Impairment Loss	(36.5)
Accumulated impairment losses	(36.5)	0.0	$ 0.0
Beginning balance	395.9	350.6
Goodwill acquired		4.0
Goodwill adjustment		(0.8)
Currency translation adjustment	(9.1)	42.1
Ending balance	$ 350.3	$ 395.9