Description of Business and Significant Accounting Policies - Impact of the Adoption of Topic 606 on the Condensed Consolidated Income Statements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue
Professional service revenues	$ 129	$ 120
Total revenues	861	808
Expense
Commissions expense	4
Total expense	790	758
Income before provision for income taxes	67	45
Income tax expense	13	16
Net income	$ 54	$ 29
Basic EPS (in dollars per share)	$ 0.77	$ 0.42
Diluted EPS (in dollars per share)	$ 0.75	$ 0.41
Assets
Unbilled revenue (net of advance collections)	$ 286			$ 297
Prepaid expenses and other current assets	22			15
Other assets	18			16
Accounts payable and other current liabilities	46			59
Liabilities
Other liabilities	10			14
Equity
Retained earnings	(332)			$ (377)
Balance using Previous Standard
Revenue
Professional service revenues	126
Total revenues	858
Expense
Commissions expense	13
Total expense	799
Income before provision for income taxes	56
Income tax expense	11
Net income	$ 45
Basic EPS (in dollars per share)	$ 0.64
Diluted EPS (in dollars per share)	$ 0.62
Assets
Unbilled revenue (net of advance collections)	$ 291
Prepaid expenses and other current assets	14
Other assets	14
Liabilities
Accounts payable and other current liabilities	47
Other liabilities	12
Equity
Retained earnings	(342)
ASU 2014-09 | Increase (Decrease)
Revenue
Professional service revenues	3
Total revenues	3
Expense
Commissions expense	(9)
Total expense	(9)
Income before provision for income taxes	11
Income tax expense	2
Net income	$ 9
Basic EPS (in dollars per share)	$ 0.13
Diluted EPS (in dollars per share)	$ 0.13
Assets
Unbilled revenue (net of advance collections)	$ (5)
Prepaid expenses and other current assets	8
Other assets	4
Liabilities
Accounts payable and other current liabilities	(1)
Other liabilities	(2)
Equity
Retained earnings	$ 10		$ 1