Segment and Geographic Information - Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary of financial information of segments
Number of operating segments | segment									3
Revenues	$ 552.7	$ 524.8	$ 629.4	$ 379.8	$ 373.7	$ 365.5	$ 373.9	$ 361.3	$ 2,086.7	$ 1,474.4	$ 1,405.0
Out-of-pocket reimbursements	41.2	37.8	26.8	25.7	25.1	21.2	16.6	19.4	131.5	82.3	69.0
Total revenues	593.9	562.6	656.2	405.5	398.8	386.7	390.5	380.7	2,218.2	1,556.7	1,474.0
Costs and expenses	481.3	472.1	525.2	326.4	296.5	289.6	320.4	306.9	1,805.0	1,213.4	1,150.2
Depreciation and amortization	35.3	34.7	34.8	23.2	26.9	22.8	24.2	22.1	128.0	96.0	91.1
Operating income (loss)	77.3	55.8	96.2	55.9	75.4	74.3	45.9	51.7	285.2	247.3	232.7
Other income (loss), net	1.8	8.7	15.5	193.0	2.7	6.7	7.0	6.3	219.0	22.7	204.5
Gain on sale of business					5.5	0.0	0.0	0.0	0.0	5.5	0.0
Equity in earnings of unconsolidated affiliates	27.2	4.2	3.9	19.2	3.3	7.0	10.2	6.7	54.5	27.2	45.4
Earnings from continuing operations before interest, income taxes and non-controlling interest									558.7	302.7	482.6
Interest expenses	7.1	$ 6.9	$ 6.9	$ 5.9	5.5	$ 5.4	$ 6.5	$ 6.1	26.8	23.5	23.8
Assets	2,938.2				2,771.8				2,938.2	2,771.8	2,813.2
Operating
Summary of financial information of segments
Revenues									2,086.7	1,474.4	1,405.0
Operating | Domestic Financial Services
Summary of financial information of segments
Revenues									1,130.3	937.7	936.8
Out-of-pocket reimbursements									111.8	73.0	60.7
Total revenues									1,299.0	1,068.8	1,043.8
Costs and expenses									1,072.2	829.0	792.3
Depreciation and amortization									86.8	77.3	67.7
Operating income (loss)									140.0	162.5	183.8
Other income (loss), net									234.2	13.9	207.2
Gain on sale of business										0.0
Equity in earnings of unconsolidated affiliates									34.7	14.3	23.8
Earnings from continuing operations before interest, income taxes and non-controlling interest									408.9	190.7	414.8
Assets	2,212.2				2,234.9				2,212.2	2,234.9	2,438.6
Operating | International Financial Services
Summary of financial information of segments
Revenues									537.9	110.5	91.8
Out-of-pocket reimbursements									12.2	1.2	1.7
Total revenues									550.6	112.1	95.1
Costs and expenses									449.3	98.2	86.1
Depreciation and amortization									30.7	3.1	4.8
Operating income (loss)									70.6	10.8	4.2
Other income (loss), net									(15.5)	8.7	(2.6)
Gain on sale of business										5.5
Equity in earnings of unconsolidated affiliates									19.3	12.4	21.3
Earnings from continuing operations before interest, income taxes and non-controlling interest									74.4	37.4	22.9
Assets	756.1				430.0				756.1	430.0	217.6
Operating | Healthcare Services
Summary of financial information of segments
Revenues									418.5	426.2	376.4
Out-of-pocket reimbursements									7.5	8.5	8.2
Total revenues									426.0	434.7	384.6
Costs and expenses									340.9	345.1	321.3
Depreciation and amortization									10.5	15.6	18.6
Operating income (loss)									74.6	74.0	44.7
Other income (loss), net									0.3	0.1	(0.1)
Gain on sale of business										0.0
Equity in earnings of unconsolidated affiliates									0.5	0.5	0.3
Earnings from continuing operations before interest, income taxes and non-controlling interest									75.4	74.6	44.9
Assets	524.5				552.2				524.5	552.2	405.3
Intersegment Operating/Elimination/Adjustments
Summary of financial information of segments
Revenues									(57.4)	(58.5)	(47.9)
Out-of-pocket reimbursements									0.0	(0.4)	(1.6)
Total revenues									(57.4)	(58.9)	(49.5)
Costs and expenses									(57.4)	(58.9)	(49.5)
Depreciation and amortization									0.0	0.0	0.0
Operating income (loss)									0.0	0.0	0.0
Other income (loss), net									0.0	0.0	0.0
Gain on sale of business										0.0
Equity in earnings of unconsolidated affiliates									0.0	0.0	0.0
Earnings from continuing operations before interest, income taxes and non-controlling interest									0.0	0.0	0.0
Assets	(554.6)				(517.9)				(554.6)	(517.9)	(543.5)
Intersegment Operating/Elimination/Adjustments | Domestic Financial Services
Summary of financial information of segments
Revenues									56.9	58.1	46.3
Intersegment Operating/Elimination/Adjustments | International Financial Services
Summary of financial information of segments
Revenues									0.5	0.4	1.6
Intersegment Operating/Elimination/Adjustments | Healthcare Services
Summary of financial information of segments
Revenues									0.0	0.0	0.0
Discontinued Operations, Held-for-sale | Customer Communications
Summary of financial information of segments
Assets held for sale	$ 0.0				$ 72.6				$ 0.0	$ 72.6	$ 295.2