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Segment Information - Schedule of reconciliation of segment income before tax to income from continuing operations before income taxes (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating income from continuing operations before income taxes | $ 717 | $ 413 | $ 277 | $ 441 | $ 635 | $ 417 | $ 445 | $ 469 | $ 1,848 | $ 1,966 | $ 1,671
Interest income |  |  |  |  |  |  |  |  | 14 | 10 | 9
Interest expense |  |  |  |  |  |  |  |  | (273) | (255) | (210)
Other income |  |  |  |  |  |  |  |  | 100 | 44 | 68
Income before income taxes |  |  |  |  |  |  |  |  | 1,689 | 1,765 | 1,538
Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating income from continuing operations before income taxes |  |  |  |  |  |  |  |  | 2,042 | 2,133 | 1,858
Operating Segments [Member] | Risk Solutions [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating income from continuing operations before income taxes |  |  |  |  |  |  |  |  | 1,506 | 1,648 | 1,540
Operating Segments [Member] | HR Solutions [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating income from continuing operations before income taxes |  |  |  |  |  |  |  |  | 536 | 485 | 318
Segment Reconciling Items [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating income from continuing operations before income taxes |  |  |  |  |  |  |  |  | $ (194) | $ (167) | $ (187)

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- Definition Amount of the cost of borrowed funds accounted for as interest expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.9) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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