Stockholders' Equity - Components of accumulated other comprehensive loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Unrealized gains/(losses) on investment securities:
Unrealized gains/(losses) on investment securities, beginning of period	$ 0.6	$ 11.0	$ (3.8)	$ 7.8
Unrealized gains	8.6	6.2	15.4	12.2
Tax expense	(3.1)	(2.3)	(5.5)	(4.4)
Tax expense related to reclassifications	0.1	0.3	0.1	0.7
Net unrealized gains on investment securities	5.2	3.5	9.6	6.7
Unrealized gains on investment securities, end of period	5.8	14.5	5.8	14.5
Unrealized gains/(losses) on hedging activities:
Unrealized gains on hedging activities, beginning of period	16.8	3.6	33.8	41.4
Unrealized gains/(losses)	(36.0)	20.1	(47.2)	(6.2)
Tax benefit	0.8	0.2	1.0	2.3
Tax expense/(benefit) related to reclassifications	0.0	0.4	(0.2)	1.0
Net unrealized gains/(losses) on hedging activities	(41.2)	10.7	(58.2)	(27.1)
Unrealized gains/(losses) on hedging activities, end of period	(24.4)	14.3	(24.4)	14.3
Foreign currency translation adjustments:
Foreign currency translation adjustments, beginning of period	(70.9)	(68.3)	(70.7)	(66.0)
Foreign currency translation adjustments	(2.4)	(0.6)	(2.0)	(3.9)
Tax expense/(benefit)	1.0	(1.2)	0.4	(0.2)
Net foreign currency translation adjustments	(1.4)	(1.8)	(1.6)	(4.1)
Foreign currency translation adjustments, end of period	(72.3)	(70.1)	(72.3)	(70.1)
Defined benefit pension plan adjustments:
Defined benefit pension plan adjustments, beginning of period	(120.3)	(125.4)	(122.1)	(127.1)
Tax benefit related to reclassifications	(1.1)	(0.9)	(2.1)	(1.9)
Net defined benefit pension plan adjustments	1.8	1.7	3.6	3.4
Defined benefit pension plan adjustments, end of period	(118.5)	(123.7)	(118.5)	(123.7)
Accumulated other comprehensive loss, end of period	(209.4)	(165.0)	(209.4)	(165.0)	$ (162.8)
Revenue
Unrealized gains/(losses) on investment securities:
Reclassification of gains into Revenues	(0.4)	(0.7)	(0.4)	(1.8)
Unrealized gains/(losses) on hedging activities:
Reclassification of (gains)/losses into Income Statement	(6.9)	(10.9)	(13.5)	(26.0)
Interest Expense
Unrealized gains/(losses) on hedging activities:
Reclassification of (gains)/losses into Income Statement	0.9	0.9	1.7	1.8
Cost of Services
Defined benefit pension plan adjustments:
Reclassification of losses into Cost of services	$ 2.9	$ 2.6	$ 5.7	$ 5.3