Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 500.1	$ 480.2	$ 485.1	$ 495.2	$ 487.0	$ 464.0	$ 465.5	$ 475.9	$ 1,960.6		$ 1,892.4		$ 1,744.0
Out-of-pocket reimbursements	167.2	171.6	172.0	187.2	171.6	168.0	167.3	177.3	698.0		684.2		644.7
Total revenues	667.3	651.8	657.1	682.4	658.6	632.0	632.8	653.2	2,658.6		2,576.6		2,388.7
Costs and expenses	544.8	531.4	551.3	574.6	559.8	544.4	539.6	559.1	2,202.1		2,202.9		1,997.0
Depreciation and amortization (including goodwill impairment)	42.9	34.1	33.1	33.2	99.6	40.5	42.3	34.0	143.3		216.4		131.6
Income (loss) from operations	79.6	86.3	72.7	74.6	(0.8)	47.1	50.9	60.1	313.2		157.3		260.1
Interest expense	(7.2)	(8.4)	(9.3)	(9.6)	(9.5)	(10.6)	(11.7)	(11.7)	(34.5)		(43.5)		(46.5)
Other income, net	68.2	73.4	28.4	73.2	77.2	72.4	194.2	29.7	243.2		373.5		38.7
Equity in earnings of unconsolidated affiliates	4.1	3.8	9.5	5.6	22.4	3.1	1.4	5.3	23.0		32.2		21.7
Income before income taxes and non-controlling interest	144.7	155.1	101.3	143.8	89.3	112.0	234.8	83.4	544.9		519.5		274.0
Income taxes	60.7	58.2	22.8	50.6	51.4	26.1	89.9	28.1	192.3		195.5		95.8
Net income	$ 84.0	$ 96.9	$ 78.5	$ 93.2	$ 37.9	$ 85.9	$ 144.9	$ 55.3	$ 352.6		$ 324.0		$ 178.2
Weighted average common shares outstanding	42.1	42.9	43.7	44.3	45.0	45.1	45.0	44.5	43.2		44.9		45.7
Basic earnings per share (in dollars per share)	$ 2.00	$ 2.26	$ 1.80	$ 2.10	$ 0.84	$ 1.90	$ 3.22	$ 1.24	$ 8.15	[1]	$ 7.22	[2]	$ 4.01
Weighted average diluted shares outstanding	42.6	43.5	44.5	45.6	46.1	46.0	45.7	45.2	44.1		45.8		46.3
Diluted earnings per share (in dollars per share)	$ 1.97	$ 2.23	$ 1.77	$ 2.04	$ 0.82	$ 1.87	$ 3.17	$ 1.22	$ 8.00	[1]	$ 7.08	[2]	$ 3.95
Cash dividends per share of common stock (in dollars per share)	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.40	$ 0	$ 0	$ 0.40	$ 1.20		$ 0.80		$ 0.70

[1] Earnings per share are computed independently for each of the quarters presented. Accordingly, the accumulation of 2013 quarterly earnings per share may not equal the total computed for the year.

[2] Earnings per share are computed independently for each of the quarters presented. Accordingly, the accumulation of 2012 quarterly earnings per share may not equal the total computed for the year.