Summary of Significant Accounting Principles and Practices (Details)	12 Months Ended						12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2014 GBP (£)	Dec. 31, 2013 GBP (£)	Dec. 31, 2011 USD ($)	Dec. 31, 2014 Buildings [Member]	Dec. 31, 2014 Automobiles [Member]	Dec. 31, 2014 Minimum [Member]	Dec. 31, 2014 Minimum [Member] Software [Member]	Dec. 31, 2014 Minimum [Member] Furniture, fixtures and equipment [Member]	Dec. 31, 2014 Minimum [Member] Computer equipment [Member]	Dec. 31, 2014 Maximum [Member]	Dec. 31, 2014 Maximum [Member] Software [Member]	Dec. 31, 2014 Maximum [Member] Leasehold improvements [Member]	Dec. 31, 2014 Maximum [Member] Furniture, fixtures and equipment [Member]	Dec. 31, 2014 Maximum [Member] Computer equipment [Member]	Dec. 31, 2014 Weighted average [Member]
Revenue Recognition
Term of outsourcing contracts for benefit services									3 years				5 years
Term of outsourcing contracts for HR BPO services									5 years				10 years
Cash and Cash Equivalents
Cash and cash equivalents and short-term investments	$ 768,000,000	$ 1,000,000,000
Restricted cash and investments, current	169,000,000	214,000,000
Operating funds in U.K.	63,000,000	126,000,000		40,500,000.0	77,000,000
Cash and cash equivalents, restricted	106,000,000	88,000,000
Fiduciary Assets and Liabilities
Premium trust balances	4,000,000,000	3,800,000,000
Allowance for Doubtful Accounts
Allowance for doubtful accounts	74,000,000	90,000,000	118,000,000			104,000,000
Fixed Assets
Fixed assets, original life, weighted-average							35 years	6 years		4 years	4 years	4 years		7 years	10 years	10 years	6 years
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets									1 year				16 years					11 years
Foreign Currency
Effect of foreign exchange gains (losses) on the consolidated statements of income	(1,000,000)	3,000,000	(16,000,000)
Derivative (loss) gain	$ (19,000,000)	$ (10,000,000)	$ 3,000,000