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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue
Revenues	$ 850	$ 801	$ 1,711	$ 1,608
Expense
Commissions expense	8		12
Total expense	774	744	1,564	1,502
Income before provision for income taxes	72	52	139	97
Income tax expense	14	12	27	28
Net income	$ 58	$ 40	$ 112	$ 69
Basic earnings per share (in dollars per share)	$ 0.82	$ 0.58	$ 1.59	$ 1.00
Diluted earnings per share (in dollars per share)	$ 0.80	$ 0.56	$ 1.55	$ 0.97
Assets
Cash and cash equivalents	$ 202		$ 202			$ 336
Restricted cash, cash equivalents and investments	629		629			1,280
Unbilled revenue (net of advance collections)	268		268			297
Prepaid expenses and other current assets	37		37			15
Other assets	23		23			16
Liabilities
Accounts payable and other current liabilities	39		39			59
Deferred income taxes	70		70			68
Other liabilities	16		16			14
Equity
Retained earnings	(302)		(302)			$ (377)
Professional service revenues
Revenue
Revenues	115	$ 109	244	$ 229
Balance Using Previous Standard
Revenue
Revenues	850		1,709
Expense
Commissions expense	14		27
Total expense	780		1,579
Income before provision for income taxes	66		122
Income tax expense	12		23
Net income	$ 54		$ 99
Basic earnings per share (in dollars per share)	$ 0.77		$ 1.41
Diluted earnings per share (in dollars per share)	$ 0.75		$ 1.37
Assets
Cash and cash equivalents	$ 208		$ 208
Restricted cash, cash equivalents and investments	623		623
Unbilled revenue (net of advance collections)	274		274
Prepaid expenses and other current assets	29		29
Other assets	17		17
Liabilities
Accounts payable and other current liabilities	43		43
Deferred income taxes	68		68
Other liabilities	20		20
Equity
Retained earnings	(316)		(316)
Balance Using Previous Standard | Professional service revenues
Revenue
Revenues	115		242
ASU 2014-09 | Increase (Decrease)
Revenue
Revenues	0		2
Expense
Commissions expense	(6)		(15)
Total expense	(6)		(15)
Income before provision for income taxes	6		17
Income tax expense	2		4
Net income	$ 4		$ 13
Basic earnings per share (in dollars per share)	$ 0.05		$ 0.18
Diluted earnings per share (in dollars per share)	$ 0.05		$ 0.18
Assets
Cash and cash equivalents	$ (6)		$ (6)
Restricted cash, cash equivalents and investments	6		6
Unbilled revenue (net of advance collections)	(6)		(6)
Prepaid expenses and other current assets	8		8
Other assets	6		6
Liabilities
Accounts payable and other current liabilities	(4)		(4)
Deferred income taxes	2		2
Other liabilities	(4)		(4)
Equity
Retained earnings	14		14		$ 1
ASU 2014-09 | Increase (Decrease) | Professional service revenues
Revenue
Revenues	$ 0		$ 2