Description of Business and Significant Accounting Policies - Impact of the Adoption of Topic 606 on the Condensed Consolidated Income Statements and Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue
Revenues	$ 875	$ 818	$ 2,586	$ 2,427
Expense
Commissions expense	6		17
Total expense	813	755	2,377	2,258
Income before provision for income taxes	60	58	199	156
Income tax expense	9	15	36	44
Net income	$ 51	$ 43	$ 163	$ 112
Basic earnings per share (in dollars per share)	$ 0.73	$ 0.62	$ 2.32	$ 1.62
Diluted earnings per share (in dollars per share)	$ 0.71	$ 0.60	$ 2.25	$ 1.57
Assets
Cash and cash equivalents	$ 237		$ 237			$ 336
Restricted cash, cash equivalents and investments	621		621			1,280
Unbilled revenue (net of advance collections)	306		306			297
Prepaid expenses and other current assets	38		38			15
Other assets	27		27			16
Liabilities
Accounts payable and other current liabilities	43		43			59
Deferred income taxes	72		72			68
Other liabilities	16		16			14
Equity
Retained earnings	(273)		(273)			$ (377)
Professional service revenues
Revenue
Revenues	119	$ 112	363	$ 341
Balance Using Previous Standard
Revenue
Revenues	876		2,585
Expense
Commissions expense	13		38
Total expense	819		2,398
Income before provision for income taxes	55		177
Income tax expense	8		31
Net income	$ 47		$ 146
Basic earnings per share (in dollars per share)	$ 0.68		$ 2.09
Diluted earnings per share (in dollars per share)	$ 0.66		$ 2.03
Assets
Cash and cash equivalents	$ 245		$ 245
Restricted cash, cash equivalents and investments	613		613
Unbilled revenue (net of advance collections)	314		314
Prepaid expenses and other current assets	29		29
Other assets	16		16
Liabilities
Accounts payable and other current liabilities	46		46
Deferred income taxes	70		70
Other liabilities	20		20
Equity
Retained earnings	(290)		(290)
Balance Using Previous Standard | Professional service revenues
Revenue
Revenues	120		362
ASU 2014-09 | Increase (Decrease)
Revenue
Revenues	(1)		1
Expense
Commissions expense	(7)		(21)
Total expense	(6)		(21)
Income before provision for income taxes	5		22
Income tax expense	1		5
Net income	$ 4		$ 17
Basic earnings per share (in dollars per share)	$ 0.05		$ 0.23
Diluted earnings per share (in dollars per share)	$ 0.05		$ 0.22
Assets
Cash and cash equivalents	$ (8)		$ (8)
Restricted cash, cash equivalents and investments	8		8
Unbilled revenue (net of advance collections)	(8)		(8)
Prepaid expenses and other current assets	9		9
Other assets	11		11
Liabilities
Accounts payable and other current liabilities	(3)		(3)
Deferred income taxes	2		2
Other liabilities	(4)		(4)
Equity
Retained earnings	17		17		$ 1
ASU 2014-09 | Increase (Decrease) | Professional service revenues
Revenue
Revenues	$ (1)		$ 1